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.
On Great Britain and Ireland in particular, the pagan Celts celebrated the Day of the Dead on All Hallows Day (1st November). 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 Great Britain, they embellished the tradition with their own, which is both a celebration of the harvest and of honoring the dead. Very much later, these traditions were transported to the United States, Canada and Australia.