UpdateCalendar.cs

private void UpdateCalendar()
{
    var displayedDays = Calendar.FindDescendants<CalendarViewDayItem>();
    foreach (var displayedDay in displayedDays)
    {
        if (_highlightedDates.Contains(displayedDay.Date.Date))
        {
            HighlightDay(displayedDay);
        }
        else
        {
            UnHighlightDay(displayedDay);
        }
    }
}