It all depends on the schools. Most of the districts that I have family in across the country will have their last day as Friday, December 17. Up until the 17th, there are very little crowds. In my mind the first 2 weeks of December are *THE BEST* times to go to Disney.....LOW crowds, all the decorations are up, the Christmas shows are all running, and you can catch Mickey's Very Merry Christmas Party on certain nights. I will take my kids out to go then if we live too far away to go for the day (we're in VA now, but will be moving to Tampa next year, so I don't have to worry about this for a few years while we're there

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On Saturday the 18th there will be noticeably more people....on Sunday the 19th you will think "this is getting crowded"....bright and early on Monday the 20th, the parks will be so crowded you can't move and you won't be able to do many things each day. The crowds continue to build throughout that week until it reaches the peak, the week between Christmas Day and New Year's Day. Immovable, impassable, take your pick of words, but that's how it is that week.
In all honesty, if you want to ride rides, see shows, and eat at certain places, that is not the time to go. If you only want to walk (ever so slowly) around and soak up the Christmas spirit and atmosphere, then it's an ok time to go.