This cannot be achieved so easy, because the DayTemplateSelector of RadCalendar is not a dependency object. You can workaround this by setting the DayTemplateSelector in the code-behind. I've attached a simple example demonstrating the approach. Please download the attachment and try it.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items