Hi Sheena! Welcome to the Disney Parks Moms Panel! I'm so glad that you asked about
Mickey's Very Merry Christmas Party as this special event hosts one of my favorite Disney parades!
As you can tell from your research, weekly schedules as well as the start date of this event do change from year to year. Typically October 31 has been the very last night of
Mickey's Not So Scary Halloween Party, with Mickey's Very Merry Christmas Party hosting their first celebration about a week and a half later. (I don't know about you, but Mickey and Minnie are MUCH faster than I am when it comes to taking down fall and Halloween decorations and starting to decorate for the winter holidays.) This past year we were all surprised when there was one more night of Halloween fun on November 1, so really anything can happen.
I know that doesn't nail anything down with your travel plans, but I have heard of many guests who are successfully able to attend both events by attending the Halloween party on Oct 31 and then they usually shoot for staying until November 9 or 10 as typically the first Christmas party is in that time frame. As far as actual nights of the parties they do bounce around. Usually held three or four times a week, I've noticed that usually one of those nights is a weekend night, but not always the same day of the weekend.
If anything you can't go wrong visiting Walt Disney World Resort that time of year. Disney's Hollywood Studios transforms magically overnight from fall to winter decor, so pop by on both October 31 and then again November 1 to gaze in that massive project.
I hope this helped in your decision making, Sheena, and we'll see you real soon!
~Kristen