When handling the SelectionChanged event on a Calendar with EnableMultiSelect="true", there is no way to determine the new selected date(s). The SelectedDatesEventArgs exposes the entire range of selected dates, but it does not tell which of those dates were previously selected. Add a property to the event args that would expose this.

OR

Add an attribute to the RadCalendar when EnableMultiSelect="true" to "DeselectWhenSelectingNewDates." If this is set to true, any date selection on the calendar would deselect any previously selected dates.