I'll go straight to your questions:
1. In order to show only months you would need to set the DateSelectionMode of the DatePicker to Month.\
2. You can achieve the desired by implementing a custom MonthButtonStyleSelector
in the CalendarStyle of the DatePicker. Using the selector you can return different Styles for the different month buttons depending on the present items in the DateTime collection you are using. For your convenience I have prepared a sample project which demonstrates the exact approach.
Hope this will work for you.
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