There are endless customisations possible with a WPF/Silverlight Calendar control.

You can apply similar customisations to the Calendar control used by C1DateTimePicker. These styles are pretty useful when user wants to change Font/FontWeight in a Calendar control.

Since the Calendar control is not exposed publicly by C1DateTimePicker, you need to use VisualTree to found the control and apply the style there. Check the attached sample implementing the same. The style in the sample is pretty basic but that's just for an idea.

For more advanced customisations with Calendar control, check out this link:-

http://msdn.microsoft.com/en-us/magazine/dd882520.aspx

Download Sample