Hi Erin!
Mickey's Very Merry Christmas Party is certainly a highlight for guests visiting during the Christmas holiday at Walt Disney World. Be sure and purchase your tickets soon as the last day of the party is December 22. The crowds in the parks become much greater as it gets closer to the holiday so having a special event party in the parks becomes difficult.
If you can get early dining in the park on that evening that would be great. But at this point dining probably is not available so perhaps you can find early dining at a close resort or simply choose to use the Quick Service locations. My favorites in the Magic Kingdom include Casey's and Columbia House.
The important thing to remember is that you can enter the park starting at 4pm the day of the party. Although the festivities officially don't start until 7pm this gives you extra time to dine or enjoy some of the attractions. Starting at 7pm all day guests will need to depart and only party guests will be allowed on the rides or even in the shops. Lines are minimal and you can enjoy a less crowded park until midnight.
Be sure and spend time on Main Street where it will often be snowing and hunt out the complimentary cookie and cocoa locations.
Probably the biggest highlight of the evening will be the Christmas parade so find a spot along the route early enough to enjoy this. The fireworks will follow the first parade. There will be entertainment throughout the park as well as special meet and greet options with characters that might not usually be there for day guests.
It is a very relaxing and festive night and it brings everyone into the holiday spirit. Between the music, snow, fewer crowds, and shorter lines I always make this a must do event at the holidays!