Hallowe'en (derived from "All Hallow's Eve," so called as it is the day before All Saints day) is celebrated on the night of October 31, usually by children dressing in costumes and going door-to-door collecting sweets.
If you want to know a lot more about this celebration, don't miss the following links. I promise you won't be disappointed.
Reading and listening
Idioms and proverbs
Interactive haunted house
HISTORY OF HALLOWEEN AND HOW THE CELTS PLAYED A PART IN THIS POPULAR HOLIDAY.