[APP-83] The first day of week in the Calendar based on the User Localisation

Backpack-SwiftUI/Calendar/Classes/CalendarHeader.swift

@@ -38,10 +38,11 @@ struct CalendarHeader: View {
     }
 
     private var weekdays: [String] {
-        var weekdaySymbols = calendar.veryShortStandaloneWeekdaySymbols
-        // We treat Sunday as the last day of the week, so we rotate the symbols array to match.
-        weekdaySymbols.append(weekdaySymbols.remove(at: weekdaySymbols.startIndex))
-        return weekdaySymbols
+        let weekdaySymbols = calendar.veryShortStandaloneWeekdaySymbols
+
+        // Rotate the array to start from the locale's first weekday
+        let startIndex = calendar.firstWeekday - 1 // `firstWeekday` is 1-based
+        return Array(weekdaySymbols[startIndex...] + weekdaySymbols[..<startIndex])
     }
 }
 

Backpack-SwiftUI/Calendar/Classes/Core/CalendarMonthGrid.swift

@@ -37,13 +37,12 @@ struct CalendarMonthGrid<
     private let daysInAWeek = 7
 
     var body: some View {
-        // Sunday is the first day of the week in the Calendar, so we need to offset (rotate the array) the days to
-        // make Monday the first day
-        let weekdaysOffset = calendar.component(.weekday, from: monthDate) - 2
-        // If the offset is -1 (Sunday), we want it to be 6 (the last day of the week),
-        // otherwise we keep the offset as it is.
-        // This is because we need to fill the first row with empty cells if the month doesn't start on a Monday.
-        let daysFromPreviousMonth = weekdaysOffset == -1 ? 6 : weekdaysOffset
+        let firstWeekday = calendar.firstWeekday // Locale-aware first day of the week
+        let weekdayOfMonthStart = calendar.component(.weekday, from: monthDate)
+        // Calculate the offset based on the first weekday
+        let weekdaysOffset = (weekdayOfMonthStart - firstWeekday + daysInAWeek) % daysInAWeek
+        let daysFromPreviousMonth = weekdaysOffset
+
         LazyVGrid(
             columns: Array(repeating: GridItem(spacing: BPKSpacing.none.value), count: daysInAWeek),
             spacing: BPKSpacing.lg.value