We went to Mickey's Not So Scary Halloween Party this past year on the first night and it was alot of fun. This past year, I believe it started on 9/4/09 and went till 11/1/09.
Here is some good information on it.
http://allears.net/tp/mk/mnssh.htm
We had a really good time. My kids dressed up for a little while... it was so hot, the soon shed the costumes for their shorts and t-shirts.
There are 2 great parades that night, special character meet-n-greets, as well at trick-o-treat stations all over Magic Kingdom.
It is a hard ticket event which means only those that have the special ticket are allowed in the park during that time. It costs me around $200 for our family of 4, but it was a really good night. Also, even though the party doesn't start until 7, you can use your ticket to get into the park around 4 or 4:30pm so you can get extra time in the park without using one of your other tickets.
As far as the Christmas party goes.. I haven't been, but a friend of mine goes just about every year. Here is a link for it
http://allears.net/tp/hol_mk.htm
I think it usually runs from early November to Mid December, and the decorations are amazing. It is on my to-do list, if we are ever able to get down there during that time of year.
Good luck. I think you will have fun with either.