internal void OnHeaderClick()
{
Debug.Assert(DisplayMode == CalendarMode.Year || DisplayMode == CalendarMode.Decade, "The DisplayMode should be Year or Decade");
CalendarItem monthControl = MonthControl;
if (monthControl != null && monthControl.MonthView != null && monthControl.YearView != null)
{
monthControl.MonthView.Visibility = Visibility.Collapsed;
monthControl.YearView.Visibility = Visibility.Visible;
UpdateMonths();
}
}