Thanksgiving week

It's not as bad as other weeks, but I don't think I'd choose to go during T-giving week. One of Disney's slowest (and best, imho) times is right after Thanksgiving through the week before Christmas. They've got the Christmas decorations up, the seasonal stuff is in high gear, the weather's great, and the crowds are low.

So, I'd say you're safe the Sunday after T-giving.

Go the first couple of weeks of December if you can. It's one of the best times to go as you can attend Mickey's Very Merry Christmas Party, the Candlelight Processional, the Osborne Family Spectacle of Lights, and all of the other seasonal things at WDW. Plus, the decorations are gorgeous and the crowds are non-existant!

Thanksgiving week isn't too bad with the exception of that Thursday/Friday/Saturday. Even then, the parks are never too full to limit entrance. Thanksgiving has a lot of holiday spirit and a heightened sense of happiness. On that Sun/Mon/Tues everything is usually very quiet.

I read that the Christmas decorations go up almost immediately after Thanksgiving; anyone know if they sometimes go up during?

The Christmas semi-trailers with all the resort decorations are in place now. The holiday teams will start about a week after Thanksgiving and have all the major stuff done by Thanksgiving itself. I think the first MVMCP is 11/10, so the Magic Kingdom will get dressed up soon.