I wnet last year on Thanksgiving Day and it was great. If you are trying to combine this with other parks, then I would check out when each park started its Christmas decorations. On Thanksgiving Day there was a lot of decorartions due to Grinchmas at IOA. I honeslty didn't notice anythign at US but I might have not noticed. As for the crowds before T day or the Monday after is best, but we went T day to the Mondya afterwards and saw everything, but you just had to use a bit of planning/patience. Compared to most holidays this one is not very crowded. Weather is great, too!
Jenn