Hi, Denise! Thank you for bringing your question to the Disney Parks Moms Panel.
Walt Disney World Resort theme parks can close early for many different reasons, including private events or events to recognize Cast Members. While I have not seen any announcements concerning the specific evening you mentioned, I have known of businesses and associations that rent the parks for the evening for pixie-dust-filled private parties. This may be why Magic Kingdom Park is closing early that evening.
Denise, try not to let the early closing time stop you from visiting the park that day. I have found over the years that days it closes early for seasonal parties or events are excellent times to experience Magic Kingdom Park with lower crowds, as many Guests avoid going to a park with shorter operational hours. Even with shorter hours over the course of the day, we usually fit in more rides with less wait. After Magic Kingdom Park closes, you could
visit one of the fantastic restaurants found in the resorts along the monorail, such as Chef Mickey's, ‘Ohana, or 1900 Park Fare. If you have the
Park Hopper Option on your tickets, you could hop over to Epcot to enjoy a delicious dinner around the World Showcase and close the evening with Illuminations.
I hope you have a magical vacation, Denise, and please let us know if you have any further questions.
Brandy B.