Halloween originated from the ancient Celtic festival called Samhain, celebrated around 2,000 years ago in Ireland, Britain, and parts of northern Europe. Samhain marked the end of the harvest season and the beginning of winter, a time when people believed the boundary between the living world and the spirit world was blurred, allowing the souls of the dead to return. To ward off harmful spirits, the Celts lit bonfires and wore costumes. Over time, this pagan festival was influenced by Christian traditions, notably All Saints' Day (November 1), with the preceding evening becoming known as All Hallows' Eve, and eventually Halloween. The holiday was brought to the United States by European immigrants, especially the Irish, where it evolved into the modern celebration with activities such as trick-or-treating, costume wearing, and pumpkin carving.