From my experience, that week should be pretty low crowds, with the crowds picking up that following weekend, week through Christmas.
We went to the party last year and had a fabulous time. We picked the week after the week you are choosing and it was really not crowded at all. We walked right onto the rides we wanted. There was lots of extra activities to do - dance party, shows, parades, fireworks. Those all got crowded because everyone gathers for them. You have to plan that part out so you can make sure you see everything. They also pass out cookies and cocoa.
It truly was very magical and a great way to start our holiday season. We have younger kids and a preteen and they all loved it. We actually are planning to go again this year if everything works out. I would really recommend it since you are attending during a less crowded time. If you go closer to Christmas, I guess it can get really crowded and become a lot less pleasant.