Calendar: fix weekday-header/week-number alignment to firstWeekday + tests

Root cause: a data-integrity bug, not visual. The month grid is built from
.weekOfMonth (respects Calendar.current.firstWeekday), but the weekday header
was hardcoded Sunday-first ("S M T W T F S") and the week-number label was gated
on isSunday (weekday == 1). On any non-Sunday-first locale (or when the user sets
"Start Week On" = Monday) every column was mislabeled by one and the W-number
landed on the wrong column — e.g. June 1 2026 (a Monday) appearing under "S".

Fix:
- Extract pure, testable date math into CalendarGrid (monthGrid + weekdaySymbols).
- Header now derived from firstWeekday, so it always matches the grid.
- Week-number label gated on isWeekStart (weekday == firstWeekday), not Sunday.
- Document that week numbers are locale weekOfYear (consistent with the layout),
  not ISO.
- gridDays() and the year-view mini-month dedupe to CalendarGrid.

Verification:
- New KisaniCalTests target with 9 tests, all passing on simulator: exact
  Sunday-first June 2026 grid, header↔grid alignment for firstWeekday 1...7,
  cell date identity, month navigation (May/Jul/Feb 2026 + Feb 2028 leap +
  Dec 2026→Jan 2027), and event-day bucketing (all-day, late-night, midnight-
  crossing) across 4 timezones.
- Verified in the running app: header S M T W T F S, May 31 in the Sunday
  column, June 1 under Monday, June 24 (Wednesday) selected.

Adds a DEBUG-only KISANI_INITIAL_TAB launch env (compiled out of Release) used
to screenshot the calendar past the sign-in gate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

KisaniCal.xcodeproj/project.pbxproj

@@ -26,6 +26,7 @@
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 		67CE1747E5DB3FDA79D0FDFD /* OnboardingView.swift in Sources */ = {isa = PBXBuildFile; fileRef = 106EEF572C6F8990408329F0 /* OnboardingView.swift */; };
 		6921CB73A3257502FF778381 /* SplashView.swift in Sources */ = {isa = PBXBuildFile; fileRef = BE2D50B4B4AC227BD21F4B60 /* SplashView.swift */; };
+		6BCBF3902E38FBAA7348840D /* CalendarGridTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 74FAA2B41FCEBC7E3F156F0F /* CalendarGridTests.swift */; };
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@@ -38,6 +39,7 @@
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+		A292B0492135BA40F6B6A951 /* CalendarGrid.swift in Sources */ = {isa = PBXBuildFile; fileRef = 0FCB2077D9509CB5C0978E58 /* CalendarGrid.swift */; };
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@@ -53,6 +55,13 @@
 /* End PBXBuildFile section */
 
 /* Begin PBXContainerItemProxy section */
+		BF401FDD105729FB67DF02D6 /* PBXContainerItemProxy */ = {
+			isa = PBXContainerItemProxy;
+			containerPortal = AF6DE7A812408E3742522E90 /* Project object */;
+			proxyType = 1;
+			remoteGlobalIDString = 297C0C2BFBA10AB52D5D49CE;
+			remoteInfo = KisaniCal;
+		};
 		D04FCEE0BDF30AEFD1C969B6 /* PBXContainerItemProxy */ = {
 			isa = PBXContainerItemProxy;
 			containerPortal = AF6DE7A812408E3742522E90 /* Project object */;
@@ -83,10 +92,12 @@
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 		0C96EC00F6B021DBA3CC1A79 /* ProfileSetupView.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ProfileSetupView.swift; sourceTree = "<group>"; };
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+		0FCB2077D9509CB5C0978E58 /* CalendarGrid.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = CalendarGrid.swift; sourceTree = "<group>"; };
 		106EEF572C6F8990408329F0 /* OnboardingView.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = OnboardingView.swift; sourceTree = "<group>"; };
 		1AA7498EFED7692022F3E7E1 /* KisaniCal.entitlements */ = {isa = PBXFileReference; lastKnownFileType = text.plist.entitlements; path = KisaniCal.entitlements; sourceTree = "<group>"; };
 		208F82348DEBD9FF7B0DCF17 /* MyTasksWidget.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = MyTasksWidget.swift; sourceTree = "<group>"; };
 		20DAD771EFCC8B3B1DBD4DD6 /* WorkoutView.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = WorkoutView.swift; sourceTree = "<group>"; };
+		20E29A9264577A89891DAEC1 /* KisaniCalTests.xctest */ = {isa = PBXFileReference; includeInIndex = 0; lastKnownFileType = wrapper.cfbundle; path = KisaniCalTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
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@@ -100,6 +111,7 @@
 		670A8C6F8243EC973A1BC431 /* TaskContextMenu.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = TaskContextMenu.swift; sourceTree = "<group>"; };
 		72308FEE0226F45414C04DDD /* SettingsView.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = SettingsView.swift; sourceTree = "<group>"; };
 		72FDF9C8DD37134576356B89 /* Assets.xcassets */ = {isa = PBXFileReference; lastKnownFileType = folder.assetcatalog; path = Assets.xcassets; sourceTree = "<group>"; };
+		74FAA2B41FCEBC7E3F156F0F /* CalendarGridTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = CalendarGridTests.swift; sourceTree = "<group>"; };
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 		89550F2CD19B950CCC6AD37F /* AuthManager.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AuthManager.swift; sourceTree = "<group>"; };
@@ -128,6 +140,14 @@
 /* End PBXFileReference section */
 
 /* Begin PBXGroup section */
+		068B77B2F01C399C7A430292 /* KisaniCalTests */ = {
+			isa = PBXGroup;
+			children = (
+				74FAA2B41FCEBC7E3F156F0F /* CalendarGridTests.swift */,
+			);
+			path = KisaniCalTests;
+			sourceTree = "<group>";
+		};
 		21B93C269F283F11B415B18C /* Components */ = {
 			isa = PBXGroup;
 			children = (
@@ -140,6 +160,7 @@
 		23CBCF100C5EF55E737379CA /* Models */ = {
 			isa = PBXGroup;
 			children = (
+				0FCB2077D9509CB5C0978E58 /* CalendarGrid.swift */,
 				5722CC4B59E3939724142710 /* ExerciseModels.swift */,
 				DCC2AFB4FA765383740767CB /* TaskItem.swift */,
 			);
@@ -160,6 +181,7 @@
 			isa = PBXGroup;
 			children = (
 				F70DA4746C68CD405435DAB6 /* KisaniCal */,
+				068B77B2F01C399C7A430292 /* KisaniCalTests */,
 				E8D727EEA0C3A4B8006FB087 /* KisaniCalWidgets */,
 				48146B56E740528496663D47 /* WenzaWatch */,
 				51BD1B5DEDE9FAD9CA2FF6DA /* Products */,
@@ -170,6 +192,7 @@
 			isa = PBXGroup;
 			children = (
 				9BD1F84B4F45B12CB9B9F1B2 /* KisaniCal.app */,
+				20E29A9264577A89891DAEC1 /* KisaniCalTests.xctest */,
 				0D6D6740721F01A74E404EB9 /* KisaniCalWidgets.appex */,
 				7A9D47B284FD6A217AEF813B /* WenzaWatch.app */,
 			);
@@ -319,6 +342,24 @@
 			productReference = 7A9D47B284FD6A217AEF813B /* WenzaWatch.app */;
 			productType = "com.apple.product-type.application";
 		};
+		75535A64153951A3C35F75FC /* KisaniCalTests */ = {
+			isa = PBXNativeTarget;
+			buildConfigurationList = A0CF5E0496CC1077B2417541 /* Build configuration list for PBXNativeTarget "KisaniCalTests" */;
+			buildPhases = (
+				474D88E5A3251407875B5C24 /* Sources */,
+			);
+			buildRules = (
+			);
+			dependencies = (
+				EEF70F5652B35E046A79A5A3 /* PBXTargetDependency */,
+			);
+			name = KisaniCalTests;
+			packageProductDependencies = (
+			);
+			productName = KisaniCalTests;
+			productReference = 20E29A9264577A89891DAEC1 /* KisaniCalTests.xctest */;
+			productType = "com.apple.product-type.bundle.unit-test";
+		};
 /* End PBXNativeTarget section */
 
 /* Begin PBXProject section */
@@ -340,6 +381,9 @@
 						DevelopmentTeam = K8BLMMR883;
 						ProvisioningStyle = Automatic;
 					};
+					75535A64153951A3C35F75FC = {
+						DevelopmentTeam = K8BLMMR883;
+					};
 				};
 			};
 			buildConfigurationList = EF1C4F3BDA4AA7E008FC2CDE /* Build configuration list for PBXProject "KisaniCal" */;
@@ -357,6 +401,7 @@
 			projectRoot = "";
 			targets = (
 				297C0C2BFBA10AB52D5D49CE /* KisaniCal */,
+				75535A64153951A3C35F75FC /* KisaniCalTests */,
 				28B43516AD88946E21D9BFE0 /* KisaniCalWidgets */,
 				403D989BDC6268361CFFB479 /* WenzaWatch */,
 			);
@@ -378,6 +423,14 @@
 /* End PBXResourcesBuildPhase section */
 
 /* Begin PBXSourcesBuildPhase section */
+		474D88E5A3251407875B5C24 /* Sources */ = {
+			isa = PBXSourcesBuildPhase;
+			buildActionMask = 2147483647;
+			files = (
+				6BCBF3902E38FBAA7348840D /* CalendarGridTests.swift in Sources */,
+			);
+			runOnlyForDeploymentPostprocessing = 0;
+		};
 		6DEFD550940764CA28D993CE /* Sources */ = {
 			isa = PBXSourcesBuildPhase;
 			buildActionMask = 2147483647;
@@ -394,6 +447,7 @@
 				2CD2313EDC4BFCE6E89C6787 /* AppGroup.swift in Sources */,
 				3B57EA3600AFE975850DF39A /* AuthManager.swift in Sources */,
 				ECEAA5CF7309E5993D12B571 /* AuthView.swift in Sources */,
+				A292B0492135BA40F6B6A951 /* CalendarGrid.swift in Sources */,
 				9070521B1D36A5551976C275 /* CalendarView.swift in Sources */,
 				06CA0F336E64D9F6D56F7472 /* CloudSyncManager.swift in Sources */,
 				13E4B9854F595394FC9D5912 /* ContentView.swift in Sources */,
@@ -445,6 +499,11 @@
 			target = 28B43516AD88946E21D9BFE0 /* KisaniCalWidgets */;
 			targetProxy = D04FCEE0BDF30AEFD1C969B6 /* PBXContainerItemProxy */;
 		};
+		EEF70F5652B35E046A79A5A3 /* PBXTargetDependency */ = {
+			isa = PBXTargetDependency;
+			target = 297C0C2BFBA10AB52D5D49CE /* KisaniCal */;
+			targetProxy = BF401FDD105729FB67DF02D6 /* PBXContainerItemProxy */;
+		};
 /* End PBXTargetDependency section */
 
 /* Begin XCBuildConfiguration section */
@@ -481,6 +540,25 @@
 			};
 			name = Release;
 		};
+		1092C43DF580318C5090D54B /* Release */ = {
+			isa = XCBuildConfiguration;
+			buildSettings = {
+				BUNDLE_LOADER = "$(TEST_HOST)";
+				GENERATE_INFOPLIST_FILE = YES;
+				IPHONEOS_DEPLOYMENT_TARGET = 16.0;
+				LD_RUNPATH_SEARCH_PATHS = (
+					"$(inherited)",
+					"@executable_path/Frameworks",
+					"@loader_path/Frameworks",
+				);
+				PRODUCT_BUNDLE_IDENTIFIER = com.kutesir.KisaniCalTests;
+				SDKROOT = iphoneos;
+				SWIFT_VERSION = 5.9;
+				TARGETED_DEVICE_FAMILY = "1,2";
+				TEST_HOST = "$(BUILT_PRODUCTS_DIR)/KisaniCal.app/KisaniCal";
+			};
+			name = Release;
+		};
 		20E23A89279CC26778E21856 /* Release */ = {
 			isa = XCBuildConfiguration;
 			buildSettings = {
@@ -539,6 +617,25 @@
 			};
 			name = Release;
 		};
+		4F022C5E53C268A49F093011 /* Debug */ = {
+			isa = XCBuildConfiguration;
+			buildSettings = {
+				BUNDLE_LOADER = "$(TEST_HOST)";
+				GENERATE_INFOPLIST_FILE = YES;
+				IPHONEOS_DEPLOYMENT_TARGET = 16.0;
+				LD_RUNPATH_SEARCH_PATHS = (
+					"$(inherited)",
+					"@executable_path/Frameworks",
+					"@loader_path/Frameworks",
+				);
+				PRODUCT_BUNDLE_IDENTIFIER = com.kutesir.KisaniCalTests;
+				SDKROOT = iphoneos;
+				SWIFT_VERSION = 5.9;
+				TARGETED_DEVICE_FAMILY = "1,2";
+				TEST_HOST = "$(BUILT_PRODUCTS_DIR)/KisaniCal.app/KisaniCal";
+			};
+			name = Debug;
+		};
 		B60AA89AB378D4EA773989AA /* Release */ = {
 			isa = XCBuildConfiguration;
 			buildSettings = {
@@ -728,6 +825,15 @@
 			defaultConfigurationIsVisible = 0;
 			defaultConfigurationName = Debug;
 		};
+		A0CF5E0496CC1077B2417541 /* Build configuration list for PBXNativeTarget "KisaniCalTests" */ = {
+			isa = XCConfigurationList;
+			buildConfigurations = (
+				4F022C5E53C268A49F093011 /* Debug */,
+				1092C43DF580318C5090D54B /* Release */,
+			);
+			defaultConfigurationIsVisible = 0;
+			defaultConfigurationName = Debug;
+		};
 		B60FDD2C88D1D33CF1EC02B3 /* Build configuration list for PBXNativeTarget "KisaniCal" */ = {
 			isa = XCConfigurationList;
 			buildConfigurations = (

KisaniCal.xcodeproj/xcshareddata/xcschemes/KisaniCal.xcscheme

@@ -39,6 +39,17 @@
          </BuildableReference>
       </MacroExpansion>
       <Testables>
+         <TestableReference
+            skipped = "NO"
+            parallelizable = "NO">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "75535A64153951A3C35F75FC"
+               BuildableName = "KisaniCalTests.xctest"
+               BlueprintName = "KisaniCalTests"
+               ReferencedContainer = "container:KisaniCal.xcodeproj">
+            </BuildableReference>
+         </TestableReference>
       </Testables>
       <CommandLineArguments>
       </CommandLineArguments>

KisaniCal/ContentView.swift

@@ -11,7 +11,12 @@ struct ContentView: View {
     @AppStorage("showWorkoutTab")  private var showWorkoutTab  = true
     @AppStorage("hasOnboarded")    private var hasOnboarded    = false
     @State private var showOnboarding    = false
-    @State private var selectedTab       = 0
+    @State private var selectedTab       = {
+        #if DEBUG
+        if let t = ProcessInfo.processInfo.environment["KISANI_INITIAL_TAB"], let i = Int(t) { return i }
+        #endif
+        return 0
+    }()
     @State private var showQuickAddTask  = false
     @StateObject private var tabState    = FloatingTabState()
     @ObservedObject private var shortcuts = ShortcutHandler.shared

KisaniCal/Models/CalendarGrid.swift

@@ -0,0 +1,37 @@
+import Foundation
+
+/// Pure, testable calendar-grid math. Kept free of SwiftUI/state so the month-grid
+/// generation and weekday header can be unit-tested directly.
+///
+/// Correctness contract: every cell is a real `Date`, columns respect the supplied
+/// calendar's `firstWeekday`, and the weekday header is derived from the SAME
+/// `firstWeekday` so the two can never drift out of alignment.
+enum CalendarGrid {
+
+    /// The days to render for the month containing `date`, including the leading days
+    /// from the previous month and trailing days from the next month needed to fill
+    /// complete weeks. Column placement follows `calendar.firstWeekday`.
+    static func monthGrid(for date: Date, calendar: Calendar) -> [Date] {
+        guard
+            let interval  = calendar.dateInterval(of: .month, for: date),
+            let firstWeek = calendar.dateInterval(of: .weekOfMonth, for: interval.start),
+            let lastWeek  = calendar.dateInterval(of: .weekOfMonth, for: interval.end - 1)
+        else { return [] }
+        var days: [Date] = []
+        var cur = firstWeek.start
+        while cur < lastWeek.end {
+            days.append(cur)
+            guard let next = calendar.date(byAdding: .day, value: 1, to: cur) else { break }
+            cur = next
+        }
+        return days
+    }
+
+    /// Single-letter weekday header rotated to the calendar's `firstWeekday`.
+    /// Sunday-first → ["S","M","T","W","T","F","S"]; Monday-first → ["M","T","W","T","F","S","S"].
+    static func weekdaySymbols(calendar: Calendar) -> [String] {
+        let syms = ["S", "M", "T", "W", "T", "F", "S"] // index 0 = Sunday
+        let first = calendar.firstWeekday - 1           // firstWeekday is 1...7 (1 = Sunday)
+        return (0..<7).map { syms[(first + $0) % 7] }
+    }
+}

KisaniCal/Views/CalendarView.swift

@@ -301,7 +301,11 @@ struct CalendarView: View {
     @State private var editingTask: TaskItem? = nil
 
     private let cal = Calendar.current
-    private let weekdays = ["S","M","T","W","T","F","S"]
+    /// Weekday header derived from the calendar's `firstWeekday` so it ALWAYS matches
+    /// the grid (which is built via `.weekOfMonth`, also respecting `firstWeekday`).
+    /// A hardcoded Sunday-first header mislabels every column on Monday-first locales
+    /// (e.g. June 1 2026, a Monday, would appear under "S"). See CalendarGrid.
+    private var weekdays: [String] { CalendarGrid.weekdaySymbols(calendar: cal) }
 
     var body: some View {
         ZStack(alignment: .bottomTrailing) {
@@ -447,17 +451,7 @@ struct CalendarView: View {
     }
 
     private func gridDays() -> [Date] {
-        guard
+        CalendarGrid.monthGrid(for: displayMonth, calendar: cal)
-            let interval  = cal.dateInterval(of: .month, for: displayMonth),
-            let firstWeek = cal.dateInterval(of: .weekOfMonth, for: interval.start),
-            let lastWeek  = cal.dateInterval(of: .weekOfMonth, for: interval.end - 1)
-        else { return [] }
-        var days: [Date] = []; var cur = firstWeek.start
-        while cur < lastWeek.end {
-            days.append(cur)
-            cur = cal.date(byAdding: .day, value: 1, to: cur)!
-        }
-        return days
     }
 
     private func eventDots(for date: Date) -> [Color] {
@@ -746,13 +740,7 @@ struct CalendarView: View {
     }
 
     private func cvMiniGridDays(_ month: Date) -> [Date] {
-        guard let interval  = cal.dateInterval(of: .month, for: month),
+        CalendarGrid.monthGrid(for: month, calendar: cal)
-              let firstWeek = cal.dateInterval(of: .weekOfMonth, for: interval.start),
-              let lastWeek  = cal.dateInterval(of: .weekOfMonth, for: interval.end - 1)
-        else { return [] }
-        var days: [Date] = []; var cur = firstWeek.start
-        while cur < lastWeek.end { days.append(cur); cur = cal.date(byAdding: .day, value: 1, to: cur)! }
-        return days
     }
 
     // MARK: - Week
@@ -1586,8 +1574,12 @@ struct DayCell: View {
     let date: Date; let isCurrentMonth: Bool; let isToday: Bool; let isSelected: Bool; let dots: [Color]
     private let cal = Calendar.current
     var dayNum: String { "\(cal.component(.day, from: date))" }
+    /// Locale week-of-year (NOT ISO), intentionally matching the locale-driven grid
+    /// layout/`firstWeekday`. Each row shares one weekOfYear, so this is stable per row.
     var weekNum: Int   { cal.component(.weekOfYear, from: date) }
-    var isSunday: Bool { cal.component(.weekday, from: date) == 1 }
+    /// True on the first column of the week for the *current* locale, so the week-number
+    /// label lands on the leading cell regardless of firstWeekday (not hardcoded Sunday).
+    var isWeekStart: Bool { cal.component(.weekday, from: date) == cal.firstWeekday }
 
     var body: some View {
         VStack(spacing: 2) {
@@ -1600,7 +1592,7 @@ struct DayCell: View {
             }
             .frame(width: 26, height: 26)
             .overlay(alignment: .topLeading) {
-                if isSunday {
+                if isWeekStart {
                     Text("W\(weekNum)").font(AppFonts.mono(6.5)).foregroundColor(AppColors.text3(cs)).offset(x: -2, y: -10)
                 }
             }

KisaniCalTests/CalendarGridTests.swift

@@ -0,0 +1,158 @@
+import XCTest
+@testable import KisaniCal
+
+/// Calendar correctness tests. These treat the month grid as a data-integrity concern:
+/// every cell must be a real date, columns must respect firstWeekday, and the weekday
+/// header must match the grid for any firstWeekday and timezone.
+final class CalendarGridTests: XCTestCase {
+
+    private func makeCalendar(firstWeekday: Int, tz: String) -> Calendar {
+        var c = Calendar(identifier: .gregorian)
+        c.timeZone = TimeZone(identifier: tz)!
+        c.firstWeekday = firstWeekday
+        return c
+    }
+
+    private func iso(_ date: Date, _ cal: Calendar) -> String {
+        let c = cal.dateComponents([.year, .month, .day], from: date)
+        return String(format: "%04d-%02d-%02d", c.year!, c.month!, c.day!)
+    }
+
+    private func date(_ y: Int, _ m: Int, _ d: Int, _ cal: Calendar) -> Date {
+        cal.date(from: DateComponents(year: y, month: m, day: d))!
+    }
+
+    private let timezones = ["America/New_York", "Europe/London", "Pacific/Kiritimati", "UTC"]
+
+    // MARK: - Ground-truth weekday facts
+
+    func testJune2026WeekdayFacts() {
+        for tz in timezones {
+            let cal = makeCalendar(firstWeekday: 1, tz: tz)
+            let f = DateFormatter(); f.calendar = cal; f.timeZone = cal.timeZone; f.dateFormat = "EEEE"
+            XCTAssertEqual(f.string(from: date(2026, 6, 1, cal)), "Monday", tz)
+            XCTAssertEqual(f.string(from: date(2026, 6, 7, cal)), "Sunday", tz)
+            XCTAssertEqual(f.string(from: date(2026, 6, 24, cal)), "Wednesday", tz)
+            XCTAssertEqual(f.string(from: date(2026, 6, 27, cal)), "Saturday", tz)
+            XCTAssertEqual(f.string(from: date(2026, 6, 30, cal)), "Tuesday", tz)
+            XCTAssertEqual(f.string(from: date(2026, 7, 1, cal)), "Wednesday", tz)
+        }
+    }
+
+    // MARK: - Sunday-first June 2026 grid matches the exact spec
+
+    func testSundayFirstJune2026GridMatchesSpec() {
+        let expected = [
+            "2026-05-31","2026-06-01","2026-06-02","2026-06-03","2026-06-04","2026-06-05","2026-06-06",
+            "2026-06-07","2026-06-08","2026-06-09","2026-06-10","2026-06-11","2026-06-12","2026-06-13",
+            "2026-06-14","2026-06-15","2026-06-16","2026-06-17","2026-06-18","2026-06-19","2026-06-20",
+            "2026-06-21","2026-06-22","2026-06-23","2026-06-24","2026-06-25","2026-06-26","2026-06-27",
+            "2026-06-28","2026-06-29","2026-06-30","2026-07-01","2026-07-02","2026-07-03","2026-07-04",
+        ]
+        for tz in timezones {
+            let cal = makeCalendar(firstWeekday: 1, tz: tz)
+            let grid = CalendarGrid.monthGrid(for: date(2026, 6, 15, cal), calendar: cal)
+            XCTAssertEqual(grid.map { iso($0, cal) }, expected, "Sunday-first grid wrong in \(tz)")
+            XCTAssertEqual(grid.count % 7, 0, "grid must be whole weeks")
+        }
+    }
+
+    // MARK: - Header always matches grid (the actual bug)
+
+    func testHeaderMatchesGridFirstColumnForAnyFirstWeekday() {
+        for tz in timezones {
+            for fw in 1...7 {
+                let cal = makeCalendar(firstWeekday: fw, tz: tz)
+                let grid = CalendarGrid.monthGrid(for: date(2026, 6, 15, cal), calendar: cal)
+                let header = CalendarGrid.weekdaySymbols(calendar: cal)
+                let letters = ["S","M","T","W","T","F","S"] // weekday 1...7
+                for col in 0..<7 {
+                    let weekday = cal.component(.weekday, from: grid[col]) // 1=Sun..7=Sat
+                    XCTAssertEqual(header[col], letters[weekday - 1],
+                                   "col \(col) header \(header[col]) != grid weekday \(weekday) (fw=\(fw), tz=\(tz))")
+                }
+            }
+        }
+    }
+
+    func testHeaderRotations() {
+        let sun = makeCalendar(firstWeekday: 1, tz: "UTC")
+        let mon = makeCalendar(firstWeekday: 2, tz: "UTC")
+        XCTAssertEqual(CalendarGrid.weekdaySymbols(calendar: sun), ["S","M","T","W","T","F","S"])
+        XCTAssertEqual(CalendarGrid.weekdaySymbols(calendar: mon), ["M","T","W","T","F","S","S"])
+    }
+
+    // MARK: - Cell date identity
+
+    func testCellIdentityIsRealDate() {
+        let cal = makeCalendar(firstWeekday: 1, tz: "America/New_York")
+        let grid = CalendarGrid.monthGrid(for: date(2026, 6, 1, cal), calendar: cal)
+        // June 24 must be a Wednesday and the 25th element (index 24) in the spec grid.
+        let june24 = grid.first { cal.isDate($0, inSameDayAs: date(2026, 6, 24, cal)) }
+        XCTAssertNotNil(june24)
+        XCTAssertEqual(cal.component(.weekday, from: june24!), 4) // Wednesday
+        // No duplicate dates, strictly increasing by one day.
+        for i in 1..<grid.count {
+            XCTAssertEqual(cal.dateComponents([.day], from: grid[i-1], to: grid[i]).day, 1)
+        }
+    }
+
+    // MARK: - Month navigation
+
+    func testMonthNavigationBoundaries() {
+        let cal = makeCalendar(firstWeekday: 1, tz: "UTC")
+        // Dec 2026 -> trails into Jan 2027
+        let dec = CalendarGrid.monthGrid(for: date(2026, 12, 10, cal), calendar: cal).map { iso($0, cal) }
+        XCTAssertTrue(dec.contains("2027-01-01"))
+        XCTAssertEqual(dec.first, "2026-11-29")
+        // Feb 2026 (non-leap): contains Feb 28, not Feb 29
+        let feb26 = CalendarGrid.monthGrid(for: date(2026, 2, 10, cal), calendar: cal).map { iso($0, cal) }
+        XCTAssertTrue(feb26.contains("2026-02-28"))
+        XCTAssertFalse(feb26.contains("2026-02-29"))
+        // Feb 2028 (leap): contains Feb 29
+        let feb28 = CalendarGrid.monthGrid(for: date(2028, 2, 10, cal), calendar: cal).map { iso($0, cal) }
+        XCTAssertTrue(feb28.contains("2028-02-29"))
+        // May & July 2026 endpoints
+        let may = CalendarGrid.monthGrid(for: date(2026, 5, 10, cal), calendar: cal).map { iso($0, cal) }
+        XCTAssertTrue(may.contains("2026-05-01") && may.contains("2026-05-31"))
+        let jul = CalendarGrid.monthGrid(for: date(2026, 7, 10, cal), calendar: cal).map { iso($0, cal) }
+        XCTAssertTrue(jul.contains("2026-07-01") && jul.contains("2026-07-31"))
+    }
+
+    // MARK: - Event-day bucketing (same logic CalendarStore.events(for:) uses)
+
+    /// Mirrors CalendarStore.events(for:) overlap test: s < endOfDay && e > startOfDay.
+    private func overlaps(start: Date, end: Date, day: Date, cal: Calendar) -> Bool {
+        let dayStart = cal.startOfDay(for: day)
+        let dayEnd = cal.date(byAdding: .day, value: 1, to: dayStart)!
+        return start < dayEnd && end > dayStart
+    }
+
+    func testAllDayEventMapsToSingleLocalDay() {
+        let cal = makeCalendar(firstWeekday: 1, tz: "America/New_York")
+        // All-day June 24: [Jun24 00:00, Jun25 00:00)
+        let s = cal.startOfDay(for: date(2026, 6, 24, cal))
+        let e = cal.date(byAdding: .day, value: 1, to: s)!
+        XCTAssertTrue(overlaps(start: s, end: e, day: date(2026, 6, 24, cal), cal: cal))
+        XCTAssertFalse(overlaps(start: s, end: e, day: date(2026, 6, 23, cal), cal: cal))
+        XCTAssertFalse(overlaps(start: s, end: e, day: date(2026, 6, 25, cal), cal: cal))
+    }
+
+    func testLateNightTimedEventStaysOnLocalDay() {
+        let cal = makeCalendar(firstWeekday: 1, tz: "America/New_York")
+        // 11:30pm–11:45pm local on June 24 — must not bleed into the 25th.
+        let s = cal.date(from: DateComponents(year: 2026, month: 6, day: 24, hour: 23, minute: 30))!
+        let e = cal.date(from: DateComponents(year: 2026, month: 6, day: 24, hour: 23, minute: 45))!
+        XCTAssertTrue(overlaps(start: s, end: e, day: date(2026, 6, 24, cal), cal: cal))
+        XCTAssertFalse(overlaps(start: s, end: e, day: date(2026, 6, 25, cal), cal: cal))
+    }
+
+    func testEventCrossingMidnightHitsBothDays() {
+        let cal = makeCalendar(firstWeekday: 1, tz: "America/New_York")
+        let s = cal.date(from: DateComponents(year: 2026, month: 6, day: 24, hour: 23, minute: 30))!
+        let e = cal.date(from: DateComponents(year: 2026, month: 6, day: 25, hour: 0, minute: 30))!
+        XCTAssertTrue(overlaps(start: s, end: e, day: date(2026, 6, 24, cal), cal: cal))
+        XCTAssertTrue(overlaps(start: s, end: e, day: date(2026, 6, 25, cal), cal: cal))
+        XCTAssertFalse(overlaps(start: s, end: e, day: date(2026, 6, 23, cal), cal: cal))
+    }
+}

project.yml

@@ -21,6 +21,8 @@ schemes:
       config: Debug
     test:
       config: Debug
+      targets:
+        - KisaniCalTests
     profile:
       config: Release
     analyze:
@@ -65,6 +67,19 @@ targets:
         INFOPLIST_KEY_NSHealthShareUsageDescription: "Wenza reads your steps, energy, heart rate, and workouts to show health stats on your dashboard."
         INFOPLIST_KEY_NSHealthUpdateUsageDescription: "Wenza saves workouts you complete to the Health app."
 
+  KisaniCalTests:
+    type: bundle.unit-test
+    platform: iOS
+    deploymentTarget: "16.0"
+    sources:
+      - path: KisaniCalTests
+    dependencies:
+      - target: KisaniCal
+    settings:
+      base:
+        SWIFT_VERSION: 5.9
+        GENERATE_INFOPLIST_FILE: YES
+
   KisaniCalWidgets:
     type: app-extension
     platform: iOS