We're also going during that timeframe (12/15-12/25, except for this year) and I've gotten mixed opinions. This is our first Christmas trip so I have no idea either, we're used to spring/summer crowds so I posted a while back asking people to compare June crowds with December crowds.
I got pretty mixed answers. I've heard some say that it will start to get really busy Saturday the 18th since a lot of kids will be out of school by then. I've heard some say that it won't get really busy until the 22nd or 23rd. I'm sure the crowds will get progressively worse while we're there, I just don't know when to expect it to be "really bad." I guess we'll find out in a few months, and I'll post so you can find out! It'll be great, Christmas at Disney, we can't wait!