We were in Disney last year for Xmas. I forget our exact dates, but I think we were there from Dec 22nd to Dec 27th or something like that. Yes, it was busy for sure. Tolerable? Absolutely. The only thing that was really nuts was MK on Christmas Day itself. We got to MK around 8:00am and stayed until noon or so. We used our FP+, did a few rides that didn't need FP+ and left. But we still got 4 pleasant hours in MK on Christmas Day and had a wonderful time. When we left, it was getting really busy. We went to AK, and it was busy but not overwhelmingly so. The other days were busy, but with smart use of FP+, rope drop and a few other strategies, you can and will have a wonderful time. I'm VERY sad we won't be spending Christmas 2016 there. I'm already counting down to being able to spend Christmas 2017 there.