Disney has actually changed the EMH schedule this year to make it a little more consistent, but this makes it completely different from last year. My guess would be that if you go in the early part of december, the EMH hours will look like Sept's schedule (which was also the schedule for February, March, the last half of August, etc - basically times when crowds should be low). If you go in the later part of December (near Christmas), look to the June/July schedule, and then right around Christmas, all bets are off (see Easter in April for a comparison).
Basically, the low-crowd calendar (not counting the water parks):
Sun AM = none
Sun PM = MK
Mon AM = AK
Mon PM = MGM
Tues AM = Epcot
Tues PM = none
Wed AM = none
Wed PM = AK
Thurs AM = MK
Thurs PM = none
Fri AM = none
Fri PM = Epcot
Sat AM = MGM
Sat PM = none
The high-crowd calendar:
Sun AM = Epcot
Sun PM = MK
Mon AM = MGM
Mon PM = AK
Tues AM = MK
Tues PM = Epcot
Wed AM = AK
Wed PM = MGM
Thurs AM = MK
Thurs PM = Epcot
Fri AM = MGM
Fri PM = MK
Sat AM = AK
Sat PM = none/TL (Typhoon Lagoon)
And the Christmas calendar is unkown. These are just a guess, but Disney has been much more consistent this year with EMH, so they're a stronger bet than looking at last year's schedule. Hope this helps.