Halloween is a holiday celebrated each year on October 31. Its roots can be traced back to the ancient Celtic festival of Samhain (pronounced sow-in). The Celts, who lived 2,000 years ago, mostly in the area that is now Ireland, the United Kingdom, and northern France, celebrated their new year on November 1. This day marked the end of summer and the harvest and the beginning of the dark, cold winter, a time of year that was often associated with human death.

During Samhain, it was believed that the boundary between the worlds of the living and the dead blurred, and the ghosts of the dead returned to earth to cause trouble and damage crops. To commemorate the event, Druids (Celtic priests) built huge sacred bonfires, where 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.

By 43 A.D., the Roman Empire had conquered the majority of Celtic territory. Over the course of the four hundred years that they ruled the Celtic lands, two Roman festivals 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.

In the 8th century, Pope Gregory III designated November 1 as a time to honor all saints. Soon, All Saints Day incorporated some of the traditions of Samhain. The evening before was known as All Hallows Eve, and later Halloween. Over time, Halloween evolved into a day of activities like trick-or-treating, carving jack-o-lanterns, festive gatherings, donning costumes, and eating treats.

The tradition of Halloween was carried to America by the immigrating Europeans. Some of the traditions changed a little, for example, the early colonists used pumpkins (native to America) instead of the turnips used in Europe to make their jack-o'-lanterns. Over time, Halloween has become a community-centered holiday and a major fall event in the United States and other countries around the world.