I went at that time last year (second week in Dec) and the weekdays (Wed and Thu) were pretty nice. Fairly low crowds, with full Christmas decor. Mad T Party wasn't on a nightly schedule yet, but the parade and fireworks were running nightly. By Fri through Sun, the crowds really ramped up and it was pretty busy. It was still manageable, but expect waits on rides (e.g., 10 minutes for Buzz, 30 minutes for HM, etc.). It was also worth noting that they still had a fair amount of Christmas merchandise on Wed, but by Sat they had started to run out of some items (mostly specific clothing sizes). So if you see things you like, I wouldn't wait to pick them up.