Have been Easter, Thanksgiving and Christmas time. Easter time and Christmas are similar with Christmas time still the worst-- Easter time, which probably has the same number of people around, has the advantage that some of the crowd is filling up the waterparks to capacity rather than the theme parks and Christmas is seldom warm enough to have even the one waterpark that is open fill up. Thanksgiving, which is quite crowded, is like off-season in comparison to either. If you think of MK as a three foot deep six foot wide closet, Thanksgiving is like trying to stuff eight adults into that closet; Christmas/New Years is like trying to stuff a hundred sumo wrestlers into it. Crowds, as comments usually indicate, are to a great extent a matter of personal tolerance; it never fails when discussed here that there wiil be two different people at WDW at the same time and one believes the crowds were easy or not bad at all and the other that they were intolerable.
Christmas time was somewhat lighter this year when we were there because I am unaware of any park actually closing because of crowd size which often happens during that time. New Years Eve in MK and Epcot are the two worst crowd days. Early mornings can often be somewhat light. To give you a concept of crowd size, here are some observed facts: (a) Soarin reached in the afternoon a few times a 3 hour 40 minute stand-by line; both Soarin and Test Track were usually close to 3 hours stand-by, saw the "singles only" Test Track line itself go to 80 minutes; noteworthy is that during the same time Mission Space often had no more than a 25 to 30 minute stand-by line -- apparently almost everyone now knows you get sick on that ride.; (b) popular rides at MK such as the mountains, Pan, Jungle Ride, and at AK, such as Safari and Everest had long periods daily with 2 to 2 1/2 hour standby's; (c) afternoon and evening street crowds at MK reached a thickness where it would take 35 to 40 minutes to walk (crawl basically) from Space Mountain to Splash Mountain; (d) because of narrow streets and crowds on them, there were times movement of the masses actually stopped in AK causing back-ups that you could not get around; (e) for popular rides at all parks fast passes for entire day and evening were often gone before noon.