Hi Tonya!
Mickey's Very Merry Christmas Party (MVMCP) is an amazing event and would be the perfect way to start off your magical vacation.
MVMCP is held from 7:00 p.m. - midnight on select evenings. It does require a separate special event ticket to attend. On the evenings that it is held, the Magic Kingdom closes early and only those Guests with the special event tickets are allowed to enter or remain in the park when MVMCP begins at 7:00 p.m.
I have heard from a lot of Guests that they have been allowed to enter the Magic Kingdom prior to 7:00 p.m. (sometimes as early as 4:00 p.m.) on MVMCP evenings. Please keep in mind that early entry is up to the discretion of the Party Hosts and you may not be allowed in early especially if the Magic Kingdom is really crowded on the day you attend MVMCP. If you are allowed early entry, remember the special MVMCP events don't begin until 7:00 p.m. If you decide to make dining reservations that take place earlier than the party begins, make sure you have your confirmation numbers handy (just in case) when you enter the park.
If you want to enter the Magic Kingdom earlier than 4:00 p.m. on the day you arrive and plan to attend MVMCP later that evening, I'd be prepared to purchase a separate regular theme park ticket to enter. Since you aren't going to try to go to the Magic Kingdom until after 5:00 p.m., I'd just go in on my ticket for MVMCP and would not enter on a regular ticket.
Please feel free to visit us with all of your planning questions. We're hEAR for you!
Wishing you minnie magical memories :-) Allison V.