Hi Melissa!
I'm currently counting down to my visit for
Mickey's Not-So-Scary Halloween Party as well! I'm really excited to see all of the new tricks and treats the party has to offer this year like
Disney's Not So Spooky Spectacular, the new nighttime castle projection and fireworks show. I bet it's booo-tiful!
Since you are
Walt Disney World Annual Passholders, you can enter
Magic Kingdom Park as early as you'd like. Mickey's Not-So-Scary Halloween Party does require a separate ticket to attend and there will be stations set up throughout the park closer to party time that will distribute wristbands to Guests with party tickets so you don't have to go back through the front entrance.
As far as wearing your costumes if you play early at the park? Go right ahead! We were at Magic Kingdom a couple of weeks ago on a party night during the day and there were tons of Guests already in costume. As long as you adhere by the
park costume guidelines, you are more than welcome to come early dressed to Halloween impress.
Mickey's Not-So-Scary Halloween Party is a great way to meet and greet with characters you usually don't see or characters that are in special costumes. The 2019 party map does not have them listed, but you can search Google for "2019 Mickey's Not-So-Scary Halloween Party Character Appearances" and see some unofficial fan sites who have listed which characters were out when they attended the party and where to find them. Please note these character appearances are subject to change and may not be the same ones during your party night. Since the party doesn't officially start until 7:00PM, that if officially what time the characters start appearing, but I have read that some Guests have met them a little earlier than expected. In my experience, Jack Skellington and Sally from Nightmare Before Christmas have been the most popular characters to meet, so if you are interested in seeing them during your visit, I suggest getting in line for them early. Any Cast Member can tell you where they will be located.
I do hope you have an unfrightfully spooktacular time!
Happy Not-So-Scary Halloween!
Leslie