October 10th, 1996, Hogwarts students decided to rally together to celebrate the seventeenth birthday of one Millicent Bulstrode, a sixth year Slytherin. The master of ceremonies was another Slytherin, Charlotte Pollaine, who threw a surprise party for Bulstrode in an unused classroom on the third floor.
It's a bit uncertain how what should have been a Slytherin affair became such a widespread phenomenon, but students from Gryffindor and Ravenclaw were also present, and the party was discussed in student journals from all four houses for days to follow.
The highlight of the evening was likely a tie between Pollaine hitting Bulstrode with a glitter charm as the guests shouted their congratulations, or Bulstrode's subsequent hexing of Pollaine and later many of the other party guests.
Hexing or not, feedback from the event was overwhelmingly positive. "It was the highlight of my time here at Hogwarts!" exclaimed an anonymous student.
Although everything has mostly calmed down and students are looking ahead towards the Halloween Maquerade Ball, they're still anticipating what Bulstrode and Pollaine will entertain them with next year.