Hi, Melissa!
There are a few things that go into timing out a
Walt Disney World vacation. I would say that two days is probably enough to see most things in the
Magic Kingdom, but there are a few things to consider.
The first thing is that I think doing
Mickey's Very Merry Christmas Party is a great idea. Although the party hours are shorter than a full day at the theme park, the wait times for many attractions tends to be much shorter than during the day, so you can see more in less time. You'll also probably get to experience character meet and greets that you don't normally get to see. One year, we saw all of the Seven Dwarfs!
I don't think you need to do the party two nights. The entertainment, including the parades and fireworks, are different during the party than they are during the regular theme park day. If you want to enjoy the Magic Kingdom as it is normally, you'll want to go during the day to see the show at the castle forecourt, to enjoy the normal parade, and to take advantage of attractions not open during Mickey's Very Merry Christmas Party.
Unless you have very young children, I would say you could see most of the stuff you'd want to see over the course of a full day at the Magic Kingdom, plus a whole party. You also get the best of both worlds - the thrills and excitement of the Magic Kingdom during the day, plus the magic and wonder of the Christmas season during the party.
I hope you have a wonderful upcoming trip!
Anna