Mickey's Very Merry Christmas Party is a separately ticketed holiday even that occurs at Magic Kingdom Park on select evenings in November and December. Typically "day guests" start clearing the park around 6:00 PM, allowing the special events to start for party guests at 7:00 PM.
Wait times for attractions during this event are generally low. However, there are several special holiday shows that draw large crowds, and lines to greet characters in holiday attire can be long.
Although they many not optimize well, we have a few Premium Touring Plans designed for use at Mickey's Very Merry Christmas Party. You can see them along with other "miscellaneous" touring plans here: