The Calendar has a SelectedDate property - which returns a DateTime value.
So you can get the day of the month from that as normal:
DateTime dt = myCalendar.SelectedDate;
int dayOfMonth = dt.Day;
If that doesn't work, then either the user hasn't selected a date (use
DateTime.HasValue
to check that) or he has selected multiple dates, in which case you probably want to look at the
Calendar.SelectedDates
collection instead and decide which one to use.