This is the default and expected behavior of RadCalendar since its SelectedDate cannot be assigned null, therefore it becomes the absolute possible minimal date. If you want to avoid the event when a day is deselected, please try to add the following condition:
sender, Telerik.Web.UI.Calendar.SelectedDatesEventArgs e)
(RadCalendar1.SelectedDates.Count > 0)
// execute custom logic here
That should do the trick.
the Telerik team
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