Well...
It's usually one of the slowest weeks of the year.
Seems like you may as well throw all the old stats out the window this year though. October is usually very manageable as well, but folks are reporting really large crowds. Guess the 50th marketing is paying off.
I know our school district (in AZ) is taking the first week of Jan. off for Christmas break this year and I have heard that some CA schools are doing the same (can anyone confirm?)
I would just plan on larger than normal crowds, then if attendance turns out to be really light, it'll be a bonus!
Even with really crazy crowds (we've been there between Christmas and New Years several times) you can still accomplish a lot - just takes a bit more planning and you may have to try a bit harder to find the magic - but it's still there!
Hope you have a great trip!