Hi Mark
Evening EMH do not start until January 2nd which as that's the day you are leaving, I'm assuming it's something you wouldn't be able to take advantage of anyway.
With regards to regular EMH I recommend double checking the park hours with the CM when you check-in at the resort for the latest information. I can say that last year when I was there for Christmas week they opened ALL the parks an hour earlier than they orginally said they would (8am) due to anticipated crowds however this was decided at quite short notice. Consequently, very few people were in the parks at opening which I'm assuming was down to the simple fact that word had not gotten out and not everyone knew.