Hey Christel!
Mickey's Very Merry Christmas Party is my favorite time to visit the Magic Kingdom. Not only are the crowds very low, there is a special parade, Mickey's Once Upon a Christmastime Parade, a special presentation of Wishes called Holiday Wishes: Celebrate the Spirit of the Season and like you said, extra special character meet 'n' greets.
The line-up of special characters changes from year-to-year. In the past there has been Santa Goofy, the Seven Dwarves, princesses with their princes, Captain Hook & Smee, Scrooge McDuck and Jack Skellington dressed as Sandy Claws. The list of special meet 'n' greets will become available much closer to when the parties begin and is subject to change.
By creating a plan of action, you are simply prioritizing who you want to meet the most and lining up for that #1 character well before the posted start time of the party which is 7:00 p.m. This is when the characters will first begin greeting guests.
With your party ticket you can enter the park at 4:00 p.m. This will give you plenty of time to get a special party park map which shows all the character meet 'n' greet locations, maybe ride a few rides then show up at the greet location. I will caution you that some people start to line up VERY early for their favorite character. The Seven Dwarves and Jack Skellington seem to be some of the most popular meet 'n' greets.
I hope this helps with your party planning. If you have any more questions about Mickey's Very Merry Christmas Party or any other aspect of your Walt Disney World vacation please come back and see me.
Wishing you a
*MAGICAL* vacation and a pixie-dusted day!
Sue N., IN