I've only visited for 1 December and that was this past year, so maybe some WDW Dec veterans can tell you if these are the norm or not.
This is from the paper they give you when checking in. The WORLD UPDATE December 1 - 15, 2002
MGM Hours were...
December 1 - 13.......9:00 am to 7:00 pm
December 14 ...........9:00 am to 7:30 pm
December 15 ...........9:00 am to 7:00 pm
The Parade was held daily at 2:30 pm
FANTASMIC! Hours were....
December 1 - 13..........6:00 pm
December 14...............6:00 pm and 7:30 pm
December 15...............6:00 pm
We did FANTASMIC! during the week and boy was it cold! I thankfully had those magic gloves (bought at target as a 2 pack but they sell there too). They were walking around selling hot cocoa

But it was still wonderful to see. I've now seen it in the rain, while shivering and finally this past Jan I managed to see it on a beautiful, cool but comfortable night
Enjoy!!! Hope it's not overly chilly while your there but a nice comfortable just right temp
