Disney have extended their "after hours" hard ticket event until September 2018. The "after hours" event is where you can enjoy Magic Kingdom for 3 hours after the park closes and everyone else has left. Tickets are limited and Disney advertise during this time that attractions will have little to no wait.
It's essentially pay for Extra Magic Hours (but they're not run on days when it's Magic Kingdom's turn for EMH).
The dates announced are listed below. Tickets cost $119+ tax per person.