Celtic Samhain (500 BC)
It's widely regarded that the origins of Halloween can be traced back to Celtic and Gaelic festivals that were prominent in ancient Britain and Ireland over 2,000 years ago. Oral traditions regarding such practices were only documented in the 9th century (when writing became more widely available), so it's unclear just how far back these early celebrations go. That said, it's generally agreed upon that the forerunner to Halloween is the Celtic festival of Samhain.
Celebrated as the Celts' New Year, Samhain marked the end the harvest season and the beginning of winter — a time often associated with death. With the new year falling on November 1st, the Celts believed that during the night before, the boundary between the worlds of the living (summer) and the dead (winter) would intermingle. And so, Samhain was celebrated on their New Year's Eve to honor and appease the spirits that would return to Earth.
Celtic priests, known as Druids, would construct massive bonfires so people could gather and make sacrifices in order to bless the year to come. Approaching the ceremonial fires, the Celts would adorn costumes, typically made from animal skulls and hides, while the Druids attempted to contact the passing spirits in order to make prophesies for the coming year.