Halloween is an observance celebrated on the night of October 31, most notably by children dressing in costumes and going door-to-door collecting sweets or money. It is celebrated in much of the Western world, though most common in the United States, Puerto Rico, the Republic of Ireland, the United Kingdom, Australia and Canada. Irish, Scots and other immigrants brought older versions of the tradition to North America in the 19th century. Most other Western countries have embraced Halloween as a part of American pop culture in the late 20th century.

The term Halloween, and its older spelling Hallowe'en, is shortened from All-hallow-even, as it is the evening before "All Hallows Day". In Ireland, the name was All Hallows Eve and this name is still used by some older people. Halloween was also sometimes called All Saints' Eve. The holiday was a day of religious festivities in various northern European pagan traditions, until it was appropriated by Christian missionaries and given a Christian interpretation. In Mexico November 1st and 2nd are celebrated as the Day of the Dead.
Halloween is also called Pooky Night in some parts of Ireland, presumably named after the púca, a mischievous spirit.