Pre-Christmas weekend is always very busy (in fact, I used to go to DLR on the pre-Christmas weekend every year, and I stopped after 2009 because it was getting way too crowded for my liking). A lot of people who plan to spend the full week at DLR will arrive over the weekend.
Add to that the fact that 12/20 is the first day of the "sneak peek" at Frozen Fun before it officially opens on 1/7/15. On 12/20, Olaf's Snow Fest, the Anna & Elsa meet and greet and the Sing-along will all be open in Hollywood Land in DCA. The new Arendelle addition to the Storybook Land Canal Boat ride will be debuting on or before 12/20 too.
Although many AP holders will be blocked out on 12/20 (all but the Premiums and Premier AP folks, as figment_jii pointed out), I tend to think that the sneak peek of Frozen Fun will somehow manage to bring in a lot of extra visitors anyway, as some folks won't want or need to wait until 1/7 to see it. A lot of bloggers, podcasters, etc., will head out to DLR on 12/20 too -- unless they are given any kind of a media preview on 12/19.
So, yes, unless it rains on 12/20, expect crowds!