We will have to further investigate this issue but the feature you describe is not a common task for the calendar. I think if you add a CheckBox in the ControlTemplate and TemplateBind its IsChecked to IsSelected of the Calendar Button it will work.
Both the week headers and the selectable day buttons use CalendarButtons. Eventually you can create a TemplateSelector and check the value of the data displayed in the button. The view model used there have Type property which you can use to determine what template should be used.
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