Halloween’s origins date back to the ancient Celtic festival of Samhain. The Celts celebrated their new year on November 1 since it marked the end of summer and the harvest and the beginning of the dark, cold winter (a time associated with death).

Celts believed that on the night before the new year, the boundary between the worlds of the living and the dead became blurred. On the night of October 31 they celebrated Samhain, when it was believed that the ghosts of the dead returned to earth. In addition to causing trouble and damaging crops, Celts thought that the presence of the otherworldly spirits made it easier for the Druids, or Celtic priests, to make predictions about the future. These prophecies were an important source of comfort and direction during the long, dark winter.

To commemorate the event, Druids built huge sacred bonfires, where the people gathered to burn crops and animals as sacrifices to the Celtic deities. During the celebration, the Celts wore costumes, typically consisting of animal heads and skins, and attempted to tell each other’s fortunes. When the celebration was over, they re-lit their hearth fires, which they had extinguished earlier that evening, from the sacred bonfire to help protect them during the coming winter.

In 43 A.D., the Roman Empire had conquered the majority of Celtic territory and eventually two festivals of Roman origin were combined with the traditional Celtic celebration of Samhain. The first was Feralia, a day in late October when the Romans traditionally commemorated the passing of the dead. The second was a day to honor Pomona, the Roman goddess of fruit and trees. The symbol of Pomona is the apple and the incorporation of this celebration into Samhain probably explains the tradition of “bobbing” for apples that is practiced today on Halloween.

(Source: history.com)