Hello Harry,
Using
Wingardium Leviosa for the RadCalendar popup is probably not an option :) so please increase the RadDatePicker z-index or decrease the RadMenu z-index:
http://www.telerik.com/help/aspnet-ajax/controlling-absolute-positioning-with-zindex.html
The RadMenu z-index is changed with an inline z-index style:
<telerik:RadMenu style="z-index:
4900">
The RadDatePicker z-index is changed with the ZIndex property if you are using RadControls Q2 2009 or later:
<telerik:RadDatePicker ZIndex="
7100" />
Alternatively, there are 3 ways to change the RadCalendar z-index with older versions:
http://www.telerik.com/community/forums/aspnet-ajax/calendar/datepicker-showing-up-underneath-pop-up-window.aspx#606838
or
http://www.telerik.com/community/forums/aspnet/calendar/raddatepicker-and-modalpopupextender-css-help.aspx#686393
or
http://www.telerik.com/community/forums/aspnet-ajax/calendar/raddatepicker-on-a-modal-popup.aspx#625807
Regards,
Dimo
the Telerik team