It is the Disney CM appreciation celebration event. MK closes early so CMs can have exclusive access.
It’s called the “Celebrate You” party. It’s only for CMs who are celebrating their 10,15,20, 25, etc year anniversary. They and a guest are invited to attend. The park will close promptly at 4:30 PM ET and all day guests must be gone. Not just heading out.
ETA: add details...
Dave