Hi ajenks22! I can't say which day is better, although perhaps Sunday, because the closer it gets to Christmas, the busier it gets, although having said that, perhaps not Sunday, because it may be more crowded with locals. Try sites like
Undercover Tourist maybe? They have crowd calendars.
My advice, learned the hard way, is to enter the park by 3 or 4 and eat an early dinner if you are doing anything besides counter service so that you don't end up "wasting" party time.

You will love the parks at Christmas! We always go in December, we love it then! Have fun!