You would have to provide your own custom template for the MonthViewDay element. It has visual states for Selected/Unselected. With regards to changing it for specific days, you would likely have to create your own MultiBinding (e.g. bind to the IsSelected property and the Start property and return the brush you want to use) and bind the Background of an element within your custom template. Or possibly add your own custom visual state (e.g. invoking the VSM's GoToState method) for whether something is a weekend or not but I suspect this will be a little involved as you would probably need to create an attached behavior to bind to the Start property and then trigger the state change as needed.