The hours for December are still very preliminary. Disney just defaults to 9-9 when they can't think of any good reason otherwise. Once the Disney statisticians see how many ADRs are booked, and once they get a glimpse on how many Fastpasses are taken, how many hotel rooms are occupied, they'll get a better view on how busy it should be and adjust the hours accordingly.
First week of December should be closing probably between 10:30 pm and midnight on non-party days.
Christmas week is still set to close at 10:00 pm, 12/30 and 12/31 set for Midnight. Now we know that's not what's going to happen at the end. I wouldn't be surprised if MK finally sets at 11:00 pm to midnight most nights, and with 12/30 and 12/31 closing at 1:00 am.
I'm actually interested in 1/2/18, as that's when I'm coming. The last few years, it's closed between 11:00 pm and midnight. The upcoming calendar? Yup, 9-9.
Touringplans.com made a post a couple years ago that analyzed the closing hours for the 4 major parks, looking at when the hours first got announced at 180 days proir against the time the park actually closed that day. His analysis showed that there was a better than 50/50 chance that the park hours shown for MK when they were posted 180 days out were NOT the same hours that the park was actually open. If they were changed, it was almost always to a later closing time.
http://blog.touringplans.com/2015/05/19/how-often-do-disney-world-park-hours-change/
Edit: doing some deeper number works on the TP.com stats, if you take out the 7:00 pm (now 6:00 pm) closing times, since they're always for the hard ticket events, MK's closing hours actually change closer to 2/3 of the time (64.7%) and it's almost always later. I'm guessing that TP's data shows closing time went earlier maybe 5 times, total, in 4 years worth of data.