errrk
just compared hours to our trip
Wed Dec 9 -- 9a-11p (fantasmic 2x)
Thurs Dec 10 -- 9a-11p (f! 2x)
Fri Dec 11 -- 8a - 12mid (f! 2x)
is it gonna be packed??
Traditionally, the weeks before Christmas were a relatively uncrowded time to go, but obviously Disney is not expecting that to be the case this year.

Regardless of the hours, I would say that Wednesday and Thursday should be moderately crowded (probably not too bad at all until evening) because most people will presumably have work or school to keep them busy. (Though if Disney changes the hours to 8 AM - midnight, I'll be thinking about running for the hills...)
By pretty much all logic, Friday will be rather worse than Wednesday or Thursday, especially in the evening. Locals can stop by after work and have 5 or 6 hours in the park!
Actually, since our party NEEDS to make an early start on Friday (our only day to do Fantasyland early since we don't have MM), I'm almost considering staying until close on Wednesday and Thursday and then (if we manage to see fireworks and Fantasmic those two nights!) just staying in the parks all day Friday and heading back fairly early Friday night. I know traditional logic is that the parks are at their busiest in the afternoons (hence the need for a break) but lately I've been wondering if it's a better bet to leave before the evening madness. Obviously the last hour or so the park is open is a great time to do rides, but I'm not sure I could manage to take an afternoon break and then come back into the parks at 10:30 PM....
Depending on how late your family stays out (and how early they can get up!) the fact that the parks don't tend to get so crowded until the afternoon might be kind of a good thing?
Anyway, I'll be there all those days (well, coming in Wednesday afternoon) so I'm crossing my fingers for the both of us to have low crowds!