I was there for your dates this year! We drove down on New Year's Day, and our first day in the parks was the 2nd. Crowds were fairly high that day, but nothing we couldn't manage. Weather was perfect up until our last day (of 5) and we nearly froze that day. I wore jeans and tanktops with a light sweatshirt at night. After the 2nd, crowds dropped off more and more everyday. Make sure to go to MK early in your trip if you want to see their Christmas tree, as it was gone by the time we went, despite the fact that other parks still had trees up after that. Oh well, the ones we saw were beautiful!!
Also, we saw all the nighttime shows except for Spectro. We decided not to see it, even though it was offered during our dates. We figured it would make the park crowded since it wasn't showing every night, and we'd seen it before. But we did see Wishes for the first time!