We arrived late morning on Thanksgiving Day (coming off a four-day
DCL cruise) this past year and I can report the following experiences:
1) Epcot was a madhouse on Thanksgiving Day and arriving late didn't help. TT FastPasses were gone for the day by 11:30 a.m. and posted standby times were 90 minutes+ (although actual waits were closer to 60 minutes). Walking through Mexico to get out to World Showcase was virtually impossible at times. We heard from other guests that MK was worse. Great weather (80 and sunny) likely contributed to the crowds as it brings out more FL residents.
2) MK was OK in some areas on Friday--10-15 minute standbys first thing in the morning for SPM and TMR, no wait for or HM--but by the afternoon things were a bit more dicey in Tomorrowland especially. Still, using FastPasses we were able to do SM (twice), BL, Philharmagic and lots more. Weather was great again.
3) Saturday, the weather turned much colder (60ish and windy). We did EMH at MGM and had a short (15-20 minute) standby wait at RNR. DW got FP for ToT, so we went there from RNR. ToT was having technical problems and only one set of cars was operating, so the standby line was crawling and even the FP line took awhile. That also makes the morning crowds tougher to judge, since the ToT problems affected lines at other rides too. Did an FP for IJ, walked right into MuppetVision and StarTours. We had lunch PSs at 50s Prime Time and it was crowded. They weren't taking any walk-ins. Left early to get ready for Cirque at night, so I can't say what the PM was like.
4) Back to MK on Sunday and the crowds were lighter, but still pretty healthy. Still cold.
All in all, lots of fun. Be prepared for potential variations in weather, however.