The form "Halloween" derives from Hallowe'en, an old contraction, still retained in Scotland, of "All Hallow's Eve," so called as it is the day before the Catholic All Saints holy day, which used to be called "All Hallows," derived from All Hallowed Souls. In Ireland, the name was Hallow Eve and this name is still used by some older people. Halloween was formerly 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 (along with Christmas and Easter, two other traditional northern European pagan holidays) and given a Christian reinterpretation. Halloween is also known as the Day of the Dead, and it is a day of celebration for Wiccans and other modern pagan traditions, though the holiday has lost its religious connotations among the populace at large.

Halloween is also called Pooky Night in some parts of Ireland, presumably named after the pookah, a mischievous spirit.

In the United Kingdom in particular, the pagan Celts celebrated the Day of the Dead on Halloween. The spirits supposedly rose from the dead and, in order to attract them, food was left on the doors. To scare off the evil spirits, the Celts wore masks. When the Romans invaded Britain, they embellished the tradition with their own, which is the celebration of the harvest and honoring the dead. These traditions were then passed on to the United States.

Halloween is sometimes associated with the occult. Many European cultural traditions hold that Halloween is one of the "liminal" times of the year when the spirit world can make contact with the natural world and when magic is most potent (see, for example, Catalan mythology about witches).

Anoka, Minnesota, USA, the self-proclaimed "Halloween Capital of the World," celebrates with a large civic parade.