I know they dress up for the halloween party but what about mickeys very merry christmas party?
If you can dress up, what would you wear
Apart from some little girls dressed as princesses, haven't seen any costumes.
Many people do wear seasonally designed clothing. Just remember that the temperature can be in the 80s--or the 40s! I took a really cute long-sleeved t-shirt (black, embroidered with holly leaves and berries) to wear for MVMCP last year, but it really was in the 80s at night. Didn't wear it for MVMCP, but did wear it our last full day at WDW. The jet stream dipped down and it was cooling off rapidly.
You just have to be prepared for what ever Mother Nature throws at you!