Hi Natalie!
Thanks for your question and for visiting the Disney Parks Moms Panel!
I took a look at the
Walt Disney World park calendar for
Magic Kingdom on August 20, 2019, and it DOES look like there is a
Mickey's Not-So-Scary Halloween Party scheduled for that evening. The regular park hours for Magic Kingdom on this day will be from 9 AM-6 PM.
The Party is a separate, "hard ticket" event that requires a separate paid admission beyond regular park hours. MNSSHP (The Party) runs from 7 PM until midnight.
If you really want to see the nighttime shows and fireworks, I HIGHLY recommend attending the Party. It is a blast!
Or, hopefully your itinerary allows for a Magic Kingdom visit on another night during your trip.
I hope this helps and I wish you a Magical trip!!
Chris