The festival of Samhain is a tradition that originated from the ancient Celtic people, who were spread across areas of what is now Ireland, the United Kingdom, and northern France. The festival was celebrated from the evening of October 31st, which was their New Year's Eve, to the evening of November 1st, their New Year's Day. The date marked the end of the harvest season and the beginning of winter, which was often associated with human death.
The Celts believed that the transition between the years was a bridge to the world of the dead. During Samhain, the veil between the living world and the otherworld was believed to be at its thinnest. This allowed spirits to cross over and roam freely. It was believed that the spirits needed to be appeased to ensure that the people and their livestock survived the winter.
To celebrate Samhain, the Druids (Celtic priests) would light large fires, which were also intended to help the Sun survive the winter. The Celts wore costumes, typically consisting of animal heads and skins, and they would tell fortunes. The bonfires and costumes were part of the effort to ward off or appease the wandering spirits.
When the celebration was over, they would extinguish their hearth fires and relight them with the sacred bonfire to help protect them during the coming winter.
Over time, as the Romans conquered Celtic territories, these traditions began to blend with Roman festivals like Feralia (a commemoration of the dead) and the festival of Pomona, the goddess of fruit and trees. This may be the origin of bobbing for apples, a game often played on Halloween.