‘This is the one night of the year we are supposed to be allowed out and about. It’s common knowledge that if they see us, Muggles assume we are simply fellow-Muggles in fancy dress."
-- Celestina Warbeck (DP4)
Falling on the 31st October every year, this holiday is celebrated by both the Muggle and wizarding world.
Hallowe'en is the occasion of seasonal decorations and an evening feast at Hogwarts. (PS10, CS8, PA8, GF16)
Decorations include a thousand live bats, candles in pumpkins, many flaming orange streamers swimming across the ceiling, pumpkins large as garden sheds, and the smell of baking pumpkin wafting magically through the castle (CS8)
Festivities center around the Hallowe'en Feast, which is the evening meal in the Great Hall. The entertainment has included ghost antics and once a "troupe of dancing skeletons" (CS9)
Other Halloween events:
- James and Lily Potter were killed by Voldemort on Halloween night, the same night Harry's blood protection from Lily vanquished Voldemort (PS1)
- When Harry visited Godric's Hollow and fought Nagini, he viewed the memory of the deaths of his parents through the eyes of Voldemort (DH17). On his way to the Dumbledore House where the Potters were staying, the Dark Lord saw children trick-or-treating dressed in pumpkin costumes. He thought about killing them but decided he'd rather just kill the Potters instead, which he did. But when he tried to kill baby Harry, the Killing Curse rebounded on him and blew him up. He couldn't die because he had made Horcruxes with parts of his soul. But the remaining part inside him was so mangled that a bit flew off creating Harry's lightning-shaped scar -- and allowing him to see the Halloween memory years later (DH35).
- On Harry's first Halloween at Hogwarts, Professor Quirrell (who was inhabited by the lost soul of Voldemort) let a Troll into the castle in hopes it would distract the teachers and let him steal the Philosopher's Stone ( PS10).
- In second year, Harry and his friends did not attend the Halloween Feast, but went to Nearly Headless Nick's Death Day Party in the dungeon. When they came upstairs, the Basilisk had come out of the Chamber of Secrets and petrified Filch's cat, Mrs. Norris (CS8,CS9) Also there was a message written "a foot high" on the wall (by Ginny Weasley who was possessed by Voldemort): The Chamber of Secrets Has Been Open, Enemies of the Heir Beware" (CS8)
- Sirius Black entered Hogwarts during the third year Halloween Feast, slashing the Fat Lady's portrait and breaking into Gryffindor Tower trying to find Scabbers the rat (PA9). Halloween turned into a slumber party in the Great Hall with "squishy purple sleeping bags" conjured by Dumbledore.
- Snape brought Lupin a "smoking goblet" of Wolfsbane Potion on Halloween, but Lupin did not transform that night - he attended the Feast (PA8).
- For the Triwizard Tournament, the students from Durmstrang and Beauxbatons arrived, and the Goblet was supposed to announce three winners after the Halloween Feast. Instead, Harry's name came out too, though he had never attempted to enter the contest. It was put there by Death Eater Barty Crouch Jr. who was masquerading as Professor Mad-Eye Moody (GF16, GF17, GF35).
The Ministry of Magic imposed rules to make Halloween celebrations less rowdy so wizards would attract less attention from Muggles. One year a vampire ran amok before being lured from a Muggle girl with a raw steak, and every Halloween St. Mungo's Hospital is full of people who have had spell accidents or been turned into artichokes. Celestina Warbuck was dismayed when she had to cancel her Halloween concert featuring screaming banshees as back-up singers, and many others in the Wizarding World felt the restrictions were unfair. As Dagbert Pips, proprietor of ‘Pumpkins R Us’ said: "‘I’ve spent the last five years cultivating luminous pumpkins, flying pumpkins, singing pumpkins, and exploding pumpkins. ...This is my livelihood, I’m not going down without a fight." (DP4)
Hallowe'en is a contraction of "All Hallows Eve", the evening of the 31st of October. Hallow is an Old English word for "holy" and later became equivalent to "saint". It falls the day before "All Hallows Day" (All Saints Day) and was originally dedicated to honouring the souls of the dead. It has since become associated in the Muggle world with witches, dressing up, magic and extortion of candy by threat. In the U.S. the word is spelled "Halloween."
[Okay, which of you editors wrote that about extortion of candy? -- SVA]