I went on December 26th last year with a family that hasn't been in 11 years. We got on Buzz (FP), Peoplemover, Tea Cups, Little Mermaid (FP), Philharmagic, Thunder Mountain (FP), Splash Mountain (80 mins), Tiki Room, Pirates (FP), and Pooh (FP). We did all that plus watch the Christmas Parade, TS meal, hours of shopping, sword and the stone show, and left at 8:30pm before the fireworks. Was it the perfect Disney day? Of course not. However, this was one of the busiest days of the year, we had an awesome day, and they can't wait to go back.
Disney does not have the capacity that it should, but it is not nearly as bad as people on these forums make it out to be. There was no 25 minute line for the restroom, QS only took about 15 minutes to get food (we did not mobile order), and we rode many smaller attractions without FP which had waits under 30 minutes. I would 100% go during the craziest day of the year if that was the only day that fit my schedule.