You don't have to wait until the official start time to get into the Magic Kingdom. The past two years we arrived between 4:00 and 4:30 pm. You'll get a wrist bracelet to show that you have MCMCP tickets. You can watch Wishes before the park finishes the regular hours just after 6:00 pm.
Tip: If you want to spend that day at the pool, resting, visiting resorts, whatever, all you need is the MVMCP ticket to spend 6 hours at MK. We never use of Hopper tickets on MVMCP days.
There's a character version of The Night Before Christmas at the theater in Tomorrowland, and the special castle show is on several times as well. There are early and later Christmas parades plus special fireworks in between.
At the Pinocchio theater (character greeting area near the Pinocchio restaurant) kids can dance with various characters. The music is fun, and adults dance, too.
The park really empties out after the second parade. Then there are no or next-to-no lines for any of the rides. We've had our own log at Splash Mountain, and we've been very alone in the Haunted Mansion. The only attraction that always seems to have a line is Peter Pan possibly because of the loading method and limited number of simultaneous riders.
Some adults and kids dress up in holiday sweaters etc. Some of the little girls wear their princess costumes. And there are a variety of Disney holiday hats, headbands, Mickey ears to be purchased.
There's a special collector pin available for purchase a new pin each night and only available during the party hours. Usually there are special plush toys, too, that are exclusive to the party times.
We love MVMCP ours is November 12 this year.