The RadDatePicker controls expose the following server-side events:
ChildrenCreated occurs when the child controls (the input area, popup buttons, and embedded calendar) are created.
SelectedDateChanged occurs when the user changes the value of the control, either when input area loses focus after the user has typed a new value, or when the user selects a new value in the popup calendar or time view control.
In order for the server-side events to be fired the RadDatePicker should trigger postback. You need to set the AutoPostBack property to true.