The night of October 31, celebrated by children who dress in costume and solicit treats door-to-door.
Halloween is traced back to many harvest and "end of summer" celebrations. It is a mishmash of many different traditions. From the Roman feast of Pomona, the Celtic festival of Samhain, and the Scottish custom of souling custom (which involves carving turnips into lanterns as a way of remembering the souls held in purgatory).
All Hallows Eve is first recorded in 1556 as a celebration the night before All Hallows Day (a much older tradition).