Hi, Laura!
Mickey's Very Merry Christmas Party is a very popular special event held annually in Magic Kingdom Park. It's a favorite of many Guests visiting during the holiday season! My family attended the 2012 party last December and I have to say that the special parade and fireworks are a real treat to behold! This year's party will be held on select nights from November 8 to December 20.
The official start time of the party is 7:00 PM; however, for the past several years, Disney has typically allowed Guests with party tickets to enter Magic Kingdom as early as 4:00 PM. It doesn't matter whether you'll be staying on property or off, as long as you've purchased tickets to the party.
If you are certain of your vacation dates and have already made your resort reservation and travel arrangements, I recommend pre-purchasing tickets to Mickey's Very Merry Christmas Party for two reasons: first, there is an advance purchase discount and, second, several party dates can and do sell out (which means you run the risk of missing the party on the evening you had planned to attend).
I hope this information is helpful to you. Come visit us again here at the Moms Panel with any other questions. We love to help plan
magical vacations!