Hi Thea,
You're going to Disney World during my MOST FAVORITE time of year! I'm so excited for you! The weather is amazing, and all of Disney property is decorated in its holiday best. It's really gorgeous!
Mickey's Very Merry Christmas Party is so much fun. My family and I attend every year and it has become a true family tradition. You're going to love it, I can promise you that. If you are already in Magic Kingdom that day and have your separate event ticket for the Christmas party, you don't have to leave and re-enter the park.
I do want to mention that Guests with tickets to Mickey's Very Merry Christmas Party are allowed to enter the park at 4:00pm the day of the event even though the ticket says the party begins at 7:00. I'm letting you know just in case you want to save your park ticket for another day or park.
One more thing...the cookies and hot chocolate at the party are awesome-don't miss that! Oh! Main Street towards the end of the evening...it SNOWS!!!!!
Wishing you the merriest and most pixie dusted vacation ever!