Discussion here:
https://forums.wdwmagic.com/threads/mickey’s-not-so-scary-halloween-party-2018.937336/
But to address your questions:
What goes on there during the celebration?
Info
here.
When does it start and when does it end?
7:00 pm to Midnight. You can enter as early at 4:00pm.
Is there a separate admission fee?
Yes. $79 - $125. It increases as Halloween gets closer, the most expensive being on Halloween ($125). See
https://disneyworld.disney.go.com/events/mickeys-not-so-scary-halloween-party/purchase/ for tickets.
What parks have special events?
- Magic Kingdom: Mickey's Not So Scary Halloween Party and Mickey's Very Merry Christmas Party (separately ticketed)
- Hollywood Studios: Star Wars Galactic Nights: Usually held around the premiere of the latest Star Wars movie, the last being on May 27 (separately ticketed)
- Epcot: Festival dujour (there is always one going on): Included with ticket price
- Animal Kingdom: None.
I've heard that costumes are allowed, what are the rules for what's allowed and what isn't?
Info
here