I've been the week before Christmas both last year and the year prior (leaving Christmas Eve in 2013 and 12/23 on 2014) and have to say that, although the crowds picked up around 12/22 and 12/23, the days before that were much less crowded than I'd ever imagined they'd be. (I've usually only gone in August though--honestly it felt SO much less crowded than I'd ever seen in late August. Later in the evening was almost walk-on in the Magic Kingdom around 12/17.) Given the way the weekends fall, I'd maybe expect the crowds to pick up on 12/19 this year, but most kids aren't out of school before that and the parks are great. Expect crowds to pick up substantially over the course of your stay and make sure you get in at least one great day in the Magic Kingdom early in your stay to make the most out of the lighter crowds and shorter lines. Later in your stay try to focus on enjoying the atmosphere (e.g. Osborne Lights--did them the last nights of our stay each year and can honestly say they were crowded but NOT Wishes/castle-lighting scary-crowded at all) and don't expect to get ANY last minute ADRs or high-demand FP+ later in your stay. So have those plans for the last few days of your stay as ironed out as you can ahead of time.