Hi Wayne!
The Magic Kingdom is beautiful all year round, but it is particularly beautiful during the holidays. It is definitely worth seeing, especially the lights on Cinderella Castle!
Your admission ticket to the Magic Kingdom will give you access to the park during it's normal operating hours that day, which typically includes hours at night. The only exception to not being able to enjoy the park at night is when the Magic Kingdom's normal hours end relatively early because it is holding a special hard ticketed event in the evening, meaning that you will need another ticket for the special event. This is different than when the Magic Kingdom closes early because it's not a busy time of the year (when it just closes early, there are no guests inside and it is closed); however, in the case of a special event, those guests with an event ticket are inside while everybody else is asked to leave.
So the question is: is there anything special going on that night that would cause the Magic Kingdom to "close early"? At that time of year, the special event offered on select evenings in the Magic Kingdom is
Mickey's Very Merry Christmas Party. It is a lot of fun and if you have never experienced it before, you should. However, it's been my experience that the party has not been offered on Saturdays. December 11, 2010 is a Saturday. Remember that this is subject to change, especially since the dates for Mickey's Very Merry Christmas Party for 2010 has not been released yet.
Enjoy planning and write back if you have any more questions!