Last year I believe the general opening time for most parks, most days that week was 8:00am instead of the usual 9:00.
The EMH schedule was the same as usual except they changed Wed & Thurs(Christmas Eve & Christmas). Therefore Wed had EMH at MK and Thurs had it at Epcot. From what we heard this was done because they expect big crowds at MK on Christmas day so they wanted to divert some crowds away from MK.
I should note that on days when the park opened at 8:00 and they had EMH, that began at 7:00.
With that said, Christmas Eve is a Fri this year with Christmas obviously being Sat. Since neither of these days are MK's EMH day, its anyones guess whether they will change anything or not.
Good luck!