Early December -- those days are better, though not necessarily slow, and the crowds won't necessarily be light. The crowds are not really light for very long during any part of the holiday season -- the holiday season is immensely popular. However, there are periods of time within the season that are less busy/less crazy than other times, and a bit more manageable and tolerable.
As this is the year of the 60th anniversary, and the 60th-specific events are running all year long (along with any added surprises thrown into the mix), I would expect there to be healthy crowds in the parks throughout most of the year.
The first couple of weeks of December have gotten much busier in the last couple of years because people caught on to the fact that it was a slightly less crowded time to go, and they piled into the parks! The fact still remains that the early December time frame falls right after Thanksgiving break and before most people take a Christmas break. Lots of folks are back at work and school in early December, but by mid-December, as Christmas breaks begin to kick in one by one, more people begin to arrive at the parks.
I still choose to go in early December over mid-November, and the crowds have not yet gotten to the unbearable stage (for me). I've been there in mid-November too, and it can be quite crowded as well. But there are certain decorations and possibly even certain holiday events that may not appear until Thanksgiving and beyond. If I want to see the Candlelight Ceremony, for example, I have to wait until the first weekend in December. Everything is just a bit more Christmasy in December than it is in November.
