Add weeknumber header and also add a sample to ControlCatalog

samples/ControlCatalog/Pages/CalendarPage.xaml

@@ -48,6 +48,12 @@
         <Calendar Name="BlackoutDatesCalendar"
                   SelectionMode="SingleDate" />
       </StackPanel>
+      <StackPanel>
+        <TextBlock Text="Weeknumbers"/>
+        <Calendar SelectionMode="SingleDate"
+                  ShowWeekNumbers="True"
+                  WeekNumberHeader="CW" />
+      </StackPanel>
     </WrapPanel>
   </StackPanel>
 </ContentPage>

src/Avalonia.Controls/Calendar/Calendar.cs

@@ -386,6 +386,23 @@ namespace Avalonia.Controls
             set => SetValue(WeekNumberRuleProperty, value);
         }
 
+        /// <summary>
+        /// Defines the <see cref="WeekNumberHeader"/> property.
+        /// </summary>
+        public static readonly StyledProperty<object?> WeekNumberHeaderProperty =
+            AvaloniaProperty.Register<Calendar, object?>(nameof(WeekNumberHeader));
+
+        /// <summary>
+        /// Gets or sets the content displayed in the week-number column header cell.
+        /// Set this to a localized string such as <c>"CW"</c>, <c>"KW"</c>, or <c>"Wk"</c>
+        /// to give users context for the week-number column. Defaults to <c>null</c> (blank).
+        /// </summary>
+        public object? WeekNumberHeader
+        {
+            get => GetValue(WeekNumberHeaderProperty);
+            set => SetValue(WeekNumberHeaderProperty, value);
+        }
+
         public static readonly StyledProperty<CalendarMode> DisplayModeProperty =
             AvaloniaProperty.Register<Calendar, CalendarMode>(
                 nameof(DisplayMode),
@@ -2244,6 +2261,7 @@ namespace Avalonia.Controls
             DisplayDateEndProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnDisplayDateEndChanged(e));
             ShowWeekNumbersProperty.Changed.AddClassHandler<Calendar>((x, _) => x.UpdateMonths());
             WeekNumberRuleProperty.Changed.AddClassHandler<Calendar>((x, _) => x.UpdateMonths());
+            WeekNumberHeaderProperty.Changed.AddClassHandler<Calendar>((x, _) => x.UpdateMonths());
             KeyDownEvent.AddClassHandler<Calendar>((x, e) => x.Calendar_KeyDown(e));
             KeyUpEvent.AddClassHandler<Calendar>((x, e) => x.Calendar_KeyUp(e));
         }

src/Avalonia.Controls/Calendar/CalendarItem.cs

@@ -639,7 +639,10 @@ namespace Avalonia.Controls.Primitives
             }
 
             if (_weekNumberHeaderLabel != null)
+            {
+                _weekNumberHeaderLabel.Content = Owner?.WeekNumberHeader;
                 _weekNumberHeaderLabel.IsVisible = show;
+            }
         }
 
         internal void UpdateYearMode()

tests/Avalonia.Controls.UnitTests/CalendarTests.cs

@@ -441,6 +441,21 @@ namespace Avalonia.Controls.UnitTests
             Assert.False((bool)field.GetValue(calendarItem)!);
         }
 
+        [Fact]
+        public void WeekNumberHeader_Defaults_To_Null()
+        {
+            var calendar = new Calendar();
+            Assert.Null(calendar.WeekNumberHeader);
+        }
+
+        [Fact]
+        public void WeekNumberHeader_Can_Be_Set_To_String()
+        {
+            var calendar = new Calendar();
+            calendar.WeekNumberHeader = "CW";
+            Assert.Equal("CW", calendar.WeekNumberHeader);
+        }
+
         // --- Week number tests ---
 
         [Fact]