I've battled peak summer crowds, and they were a walk in the park compared to Christmas week. I found it almost difficult to have a good time. During the afternoon parade, it was literally impossible to get from Liberty Square to Frontierland. All of the walkways were at a standstill and you had no choice but to stay and watch the parade (yeah, it was nice and all, but instead of watching it for the 2nd time, I wanted to ride Splash!). After the parade passed, the mob scene was actually scary.
I would try either the first or second week of December or the 2nd or 3rd week of January if you can help it.