Greetings, Carmen!
Predicting Disney crowds is much like predicting Florida weather--almost impossible, however, I'm thrilled to share my experiences with you and let you know how I'd plan our trip time.
If you are staying at a Walt Disney World Resort Hotel and can take advantage of
Extra Magic Hours, then I would definitely choose Friday for your Magic Kingdom Park visit. Even though the park will be closing at 6 PM for the evening's Mickey's Very Merry Christmas Party, I think you'll really enjoy the extra fun from the morning hours and find the park a little less crowded than the Saturday alternative. If you aren't staying at a Disney Resort it makes the decision a little trickier. Saturday will more than likely be a little more crowded, but Magic Kingdom Park will be open later, so you'll be able to spread out your day more. I would probably choose Saturday, and take a slower pace.
Have a magical time, Carmen, and please let us know if there is anything else we are able to assist you with.
Pixie Dust,
Lauren