Hello Elizabeth! Welcome to the Disney Parks Moms Panel.
My family just returned from a Walt Disney World Resort vacation that included
Mickey's Not So Scary Halloween Party after Advanced Dining Reservations at
Be Our Guest Restaurant. We booked our Advanced Dining Reservations for 5:30pm and were able to enter Magic Kingdom using only our
Special Event Tickets.
Walt Disney World Resort does allow Guests to enter Magic Kingdom before the official 7pm scheduled start time. There is no "official" time that Guests are allowed to enter, however. Basically, Cast Members will set up a special queue for Mickey's Not So Scary Halloween Party at the Main Ticket Gates sometime before the Special Event. While early entry is a common occurrence, there is no set time for Guests and early entrance.
From personal experience, scheduling anything for just after 5pm has served me well when making decisions in regard to Mickey's Not So Scary Halloween Party or
Mickey's Very Merry Christmas Party. Just be sure to enter the main gates through the Mickey's Not So Scary Halloween Party queue set up for early entrance.
Have a Spook-tacular Experience!
~Todd