Hi, Gail! Mickey's Very Merry Christmas Party is what is called a "hard ticket" event, meaning that you need a separate ticket to attend the party. The Magic Kingdom does close to guests without Mickey's Very Merry Christmas party tickets, and they distinguish guests for the party by a wristband. If you enter the theme park with a party ticket, you will be given a wristband that will allow you to stay in the park while other guests are ushered out. I have also had them ask to see the wristband before getting on some of the rides, at least in the early part of the party.
The ticket for the Christmas party actually allows you to enter the Magic Kingdom at four o'clock, which gives people the opportunity to have dinner before the party. You go through a specially marked entrance, where your wristband will be given to you as you enter.
My girls love this party! The Christmas parade on Main Street is just beautiful, and the holiday Wishes fireworks display is amazing to behold. In years past, they have offered complimentary holiday cookies and hot cocoa, which my girls enjoy while we wait for the parade. There are other holiday themed shows that you won't see during the day at the Magic Kingdom, and because a limited number of tickets are sold to the event, you don't have to worry about being overwhelmed by crowds!
For my family, this is a full day in the theme park for us. We'll sleep late that morning, lounge by the pool, and take an afternoon nap so that we can be at the park at four o'clock and not have to worry about the kids getting too tired to stay until midnight.
If you go, I hope you have a wonderful time! Enjoy your upcoming trip to Walt Disney World!