Crowds build and build starting 12/26, with 12/31 being the most crowded. We go every year at Easter, but it is no comparison to Xmas week. People were staked out along the EPCOT fence on 12/31 at Noon for the Midnight fireworks!!!
The week between Christmas and New Years Eve is the busiest week of the year.
This is a picture from 2006 in the morning of Dec 27th. It goes downhill each day thereafter. MK does close becuase of capacity limitations during that week.