You can check the following blog post showing how you can easy add your custom SelectedDates dependency property: http://www.telerik.com/blogs/using-the-multiple-selection-of-the-radcalendar-control-for-silverlight-and-wpf-with-mvvm .
When DateSelectionMode is set to Month and SelectionMode is set to Multiple or Extended selecting multiple months should be available.
RadCalendar should have property which switches between single raise of the SelectionChanged event when multiple items are selected and multiple raises (on each element hover) when multiple items are selected. When the property is in single raise mode, the event should be raised only once on MouseLeftButtonUp. In this case the SelectedItems collection should be also updated at once.
Currently, the maximum that is displayed in the calendar is 1/1/3000 and the minimum is 1/1/1000. However, you can set a value that is bigger or smaller in the SelectedDate property. Introduce an API that allows changing those dates.
Introduce a mechanism for enabling/disabling the animation executed when changing the views - month, year, etc. Currently, there is an animation executed by default which cannot be turned off.