Why you need to do this? If you want to display Months only then easier way is to have a list of months in Combox.
However I found something for you on msdn. Have a look here https://social.msdn.microsoft.com/Forums/en-US/7bdca56f-719e-44bf-be6d-a9600dfa8f78/wpf-datepicker-for-months-only?forum=wpf