Make sure to use ISOWeek calculation where requiered.

src/Avalonia.Controls/Calendar/DateTimeHelper.cs

@@ -140,6 +140,14 @@ namespace Avalonia.Controls
         /// <param name="firstDayOfWeek">The first day of the week.</param>
         /// <returns>The week number that <paramref name="date"/> falls in.</returns>
         public static int GetWeekOfYear(DateTime date, CalendarWeekRule rule, DayOfWeek firstDayOfWeek)
-            => CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date, rule, firstDayOfWeek);
+        {
+            // .NET's Calendar.GetWeekOfYear does not correctly implement ISO 8601 for the combination
+            // of FirstFourDayWeek + Monday: late-December dates that belong to week 1 of the next year
+            // are incorrectly returned as week 53. Use ISOWeek for that specific combination.
+            if (rule == CalendarWeekRule.FirstFourDayWeek && firstDayOfWeek == DayOfWeek.Monday)
+                return ISOWeek.GetWeekOfYear(date);
+
+            return CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date, rule, firstDayOfWeek);
+        }
     }
 }

tests/Avalonia.Controls.UnitTests/CalendarTests.cs

@@ -494,6 +494,8 @@ namespace Avalonia.Controls.UnitTests
         [InlineData(2023, 1, 2, System.Globalization.CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday, 1)]
         // 2022-12-31 is still in ISO week 52 of 2022
         [InlineData(2022, 12, 31, System.Globalization.CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday, 52)]
+        // .NET bug: 2018-12-31 is a Monday and is ISO week 1 of 2019, not week 53 of 2018
+        [InlineData(2018, 12, 31, System.Globalization.CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday, 1)]
         // US rule: week 1 always starts on Jan 1
         [InlineData(2023, 1, 1, System.Globalization.CalendarWeekRule.FirstDay, DayOfWeek.Sunday, 1)]
         [InlineData(2023, 12, 31, System.Globalization.CalendarWeekRule.FirstDay, DayOfWeek.Sunday, 53)]