I just saw this on the Anita Answer section of the AllEars site:
Jenni writes:
We will be visiting my mother in Orlando the week of Christmas, and plan to take one day to go to Magic Kingdom. My question is whether the Magic Kingdom is less busy on Christmas Day than other days that week?
Hi Jenni!
I'm sorry to have to tell you this, but you picked the absolute busiest week of the year to visit the Magic Kingdom. The park almost always reaches capacity by 11:00 a.m. every day of that week, so if you're planning on getting in at all, you'd better get there before the park opens. If you're in the area on December 24, you may have a marginally better chance of smaller crowds than you will have between Christmas Day and New Year's Day, when they are at their peak for the year.