The BlackoutDates property

You can choose ranges of dates to be non-selectable despite being displayed. You can implement the same by using the calendar's BlackoutDates property, which takes a collection of CalendarDateRange objects.

The following Calendar control will block the date range from August 1^st, 2017 to August 8^th, 2017, and August 21^st, 2017 to August 31^st, 2017:

<Calendar> 
    <Calendar.BlackoutDates> 
        <CalendarDateRange Start="8/1/2017" End="8/8/2017" /> 
        <CalendarDateRange Start="8/21/2017" End="8/31/2017" /> 
    </Calendar.BlackoutDates> 
</Calendar> 

All non-selection dates are marked by a cross, as shown in the following screenshot: