We were there this year beginning Jan. 2nd for 4 days. DL was very crowded, but the lines didn't seem too bad. DCA wasn't crowded at all. I think the next week would have been a great time to be there crowd wise, because the parks were beginning to thin out by Monday. It was very wet and cold the first few days we were there and starting to warm up later in the trip. But weather is one of those things you just can't predict that time of year.
Just be there at opening time, and have a great trip.