What: Unofficial Hogwarts Halloween Party
Who: Hosted by Sirius Black (& friends!) for 5th-7th years who seem like they'd be cool at a party (& not narc) and Quidditch players in 3rd+ year. Two prefects Sirius was annoyed by have been assigned patrols for this night and not told about the party (ONE IS Charles Whymper.)
Cost: No
Where: The Room of Requirement
Why: Poor teenage choices
When: Oct 31, at night!