We've gone everyother year at Thanksgiving since 1983 and each year the crowds have increased. This past year was because of people cancelling trips and rebooking them because of the hurricanes.
The beginning of the week is not bad, but by Wed. the crowds have grown, & by Thurs., Fri & Sat very crowded.
It's a beautiful time of the year the weather is 70's - 80's during the day, cooler in the evenings, with very little (if any) rain. But you have to be prepared for anything. This past year one evening it was so cool I had to wear sweatshirs with jackets, and I also had to break out my little gloves.
The parks are fully decorated by Thanksgiving day and the Christmas shows start.
You have to go with an open mind regarding the crowds. If one park is to crowded for you switch to another. Don't go to the park that has early entry, this seems to make the park crowded the whole day.