I was there that weekend last year and DL was completely full. In fact they had signs at the enterance and on Harbor saying as such. Now what they mean by full is open to discussion. We are passholders and I can tell you that we were able to leave the park mid afternoon and come back later in the day so I think what they mean by "full" is that they weren't letting any "new" people in that hadn't already been there earlier in the day. I know sometimes it gets "fire marshall" full meaning they don't let ANYONE back in at all but we were still able to get back in if we had already been in once that day. You just have to be aware of that if you leave. But if you are worried at all then I wouldn't leave during the day.
It WAS crowded but you can manage everrything if you use FP wisely. And what we did was went to DL on Friday and CA on Saturday as they were filming the Christmas parade at DL on Saturday. I don't know if they will be doing that again this year as they have moved up the official Christmas season to the following week so you might have it a bit easier this year.
Just know that it WILL be crowded whenever there is a 3 day weekend but it is doable!
