I think it all depends on when Christmas falls on the calendar---last year, Christmas was a Friday. We were there Dec 16-20 and the 19th and 20th were a Sat & Sun before Christmas and those two days WERE PACKED!!!! Sunday was worse. The 16-18 were pretty manageable. Thankfully, on Sunday (our getaway day) we had three great fast passes in the morning and a great lunch booked, then hopped on the Magical Express to depart around 3.
Because most schools only had school Mon & Tues (maybe Wed) last year before Christmas, I bet a lot of families take their kids out of school for a few days and get down there by Sat, spend almost a full week (or maybe thru the holiday).
This year, because Christmas is a Sunday, most families will be filing into WDW after the holiday I would bet. Sure, some people will bring their kids down there the week before, but their kids will be in school through 12/23 on Friday.