Check this site out. May give you an idea:
http://www.touringplans.com/BestDays.htm
We are going earlier in Dec. We did the Christmas thing years ago, before kids, and the crowds really wore on us, but we were there the 24th, thru 1/1. According to that website- the crowds steadily build, but for the first few days of your trip-looks managable.
Our kids are still young enough to miss a few days of school, so a quick trip to go to
MVMCP, and see decorations is their Christmas present. Hopefully th parks will be uncrowded, and we can accomplish alot during our 5 days.
As for weather- I'm packing a bit of everything. I was hoping for no big suitecases this trip since it is shorter- DH reminded me that jeans, and sweatshirts take up much more room, and shorts, and tees. UGG!!
Have a great trip.