Allie,
Thanks for the festive question!
So you can imagine, plans from Disney can always change, so until they release their party dates don't assume anything as set in stone. But for most of the time from mid-November through to just before Christmas
Mickey's Very Merry Christmas Party will occur on most every Sunday, Tuesday, Thursday, and Friday. They seem to pause for Thanksgiving weekend, as you might imagine, but otherwise, with only a few deviations, this seems to be a pretty reliable schedule of about 4 days per week.
If it is in your budget and schedule, I would highly recommend you go to this party. It is not just a way to enjoy the
Magic Kingdom with lighter crowds, but the holiday mood is so festive that you will forget you are in sunny Florida, and not the North Pole!
My favorite event not to miss is watching it snow on Main Street, U.S.A.!
Have a Magical Vacation!