Currently, you can use reflection in order to access the properties of the fast navigation popup. 

Workaround: 
this.radCalendar1.HeaderNavigationMode = Telerik.WinControls.UI.HeaderNavigationMode.Popup;

CalendarNavigationElement a = this.radCalendar1.CalendarElement.CalendarNavigationElement;
FieldInfo fi = typeof(CalendarNavigationElement).GetField("dropDown", BindingFlags.NonPublic | BindingFlags.Instance);
RadDateTimePickerDropDown dropDown = (RadDateTimePickerDropDown)fi.GetValue(a);
dropDown.PopupOpening += new RadPopupOpeningEventHandler(dropDown_PopupOpening);