Usually during busy times and holiday weekends the hours are extended. I would say the parks will be open til possibly 8, but this is an educated guess of course. I'm going in early Dec. (before the school breaks and holidays) and the parks will close at 6, as opposed to later in the month when they are much busier and will close at 8:00 and even 11:00 during the last week of the month!
check the link to their calendar site below to check the park hours. they will probably be listing January's schedule by next month.