Hi Cherry!
Great choice on doing the Mickey's Very Merry Christmas Party! In terms of Christmas parties, this is certainly one unlike any other!
Unfortunately, getting tickets for the party doesn't include a full day's park admission. The party starts at 7pm, and, in the past, Disney has allowed party guests to enter Magic Kingdom a few hours early (not sure what their policy will be this year. You may need to contact them at 407-934-7639 to see how early you can enter this year).
If, however, you already have general park admission, then you won't need to exit then re-enter just to do the Christmas party.
Have a Mickey Merry Christmas!