We went last December on a Friday, 12/16. It was a ghost town at US and IOA. I'm a native, never, ever go during high crowd times, and even our December trip was something I'd never seen crowd-wise. We did all of our favorite rides (no coasters) at both parks, had a leisurely lunch at Finnegan's, and did a ton of shopping -- had to make an extra trip back to the garage to put all the bags in the car and come back for more shopping (yes, our loved ones got tons of excellent presents!!!) between the hours of about ten a.m. and 3:30 p.m.
However, we went to MK that Saturday, 12/17, and it was a total madhouse, packed, wall-to-wall people. The park was full to capacity by early afternoon. I'm talking Mardi Gras packed, Superbowl packed -- you have no choice but to move the way the crowd moves you packed.
Wish we had just gone back to US/IOA that day.