Dec 1-20th or so is usually OK. Dec 24-Jan 2nd ish is as bad as it gets for crowds. We do it every year to hit the football bowl games and Diz, plus enjoy a bit of the warmer weather while I have off work (school). At Xmas expect 2 to 3 hours to stand-by on the biggies like TSM, Tower or Terror, etc. Use the FP system well, and you can still hit quite a few attractions in a day. Tour the hotels, very nicely decorated at Xmas - to avoid the insanity a bit.