Is Dec. 21 special?

[Nickarooney]

We're heading down for a Dec. 18-23 trip and even though we are staying at HRH and won't need them, I thought I would take a look at the Express Pass page to judge what Universal was thinking about crowd expectations during our vacation. Express pass for both parks are at the second price point for our entire stay except Tues., December 21 when it bumps up another level. Is there something happening that day that would cause a jump in crowd level, but have it go down again before Christmas Day?