The week between Christmas and New Year's is historically the busiest week of the year at WDW, so expect large crowds. MK will likely close at capacity at some point during that week -- perhaps more than once. Go early! You'll definitely need a touring plan, and and need to utilize Fast Passes when ever possible. You'll also need to make Priority Seatings for any restaurants that you particularly want to visit. This is not the week to "play it by ear" and just decide where to eat when you get hungry!
As for special events, there are all sorts of Christmas and holiday shows that will still be going on that week, with special fireworks in many different locations through the "world" on New Year's Eve.