My kids get out on the 19th, and a lot of schools will get out that day.
The 18 & 19th should be quite, with crowds buliding each day as you get closer to Christmas.
We have been there the past few years arriving a few days before Christmas, and found the crowds manageable. Having said that the parks will be busy and you will not be able to see everything. If you are on -site use EE & fast pass. Make PS's early and you should do fine. We arrive on the 28th thru the 3rd.