Tricked Or Treated
Within each city and town, you can visit the innkeeper once per hour, where you'll either be "tricked" or "treated."
If you're tricked, you'll be transformed into either a frog, human ghost, kitten, skeleton, or snake for a couple of minutes, or you'll be given a costume that you can wear. These include costumes of bats, assassins, pirates, skeletons, or wisps, and last for one hour. If you have better luck, you'll be treated and receive a treat bag containing various consumable candy items, a flimsy mask, or a wand that will allow you to transform friendly players for a limited amount of time.
[I basically copy & Pasted the Wotlk event details from way back in the day; but collecting a group of people to trick or treat with is a classic staple of Halloween]