Halloween's origins date back about 2,000 years, to the ancient Celtic festival of Samhain. The Celts lived in the cold parts of Northern Europe — in Britain, Ireland, and the north of France — and so for them, the new year began on November 1st, the end of the fall harvest and the beginning of winter. The night before the new year, on October 31st, the division between the world of the living and the world of the dead dissolved, and the dead could come to earth again. This was partly bad and partly good — these spirits would damage crops and cause sickness, but they also helped the Celtic priests, the druids, to tell the future, to make predictions about the coming year. The druids built huge bonfires, and regular people put out their own fires in their homes and crowded together around these fires, where they burned sacrifices for the gods, told each other's fortunes, and dressed in costumes — usually animal skins and heads. At the end of the celebration, they took a piece of the sacred bonfire and relit their own fires at home with this new flame, which was meant to help them stay warm through the long winter ahead.
First the Romans co-opted Samhain and combined it with their festivals, and then the Christians co-opted both the Celtic and Roman celebrations. In the ninth century, the pope decided that these pagan festivals needed to be replaced with a Christian holiday, so he just moved the holiday called All Saints' Day from May 13 to November 1. All Saints' Day was a time for Christians to honor all the saints and martyrs of their religion. The term for All Saints' Day in Middle English was Alholowmesse, or All-hallowmass. This became All-hallows, and so the night before was referred to as All-hallows Eve, and finally, Halloween.