The origin of Halloween comes from the ancient religious festival of Samhain. It was celebrated on October 31st by the Celtsin Ireland over 2,000 years ago. The day marked the end of the harvest season and the coming of the winter months. Crops and livestock were burned to create massive sacred bonfires as a sacrifice to appease Celtic deities and entice the sun to stay out longer during the coming winter.
The new year began on November 1st and Samhain was the night in which the barrier between the worlds of living and the dead became blurred. The Celts believed that on Samhain night, the dead returned to cause trouble, damage crops, and possess the bodies of humans. Celts used animal skins to dress up like deer, wolves, or ghouls in an attempt to avoid the spirits. Druids, the spiritual leaders of the Celts, would look through the portal of the dead to make predictions about who would marry, who would have good fortune, and who would die during the dark, cold winter.
Nearly all of Ireland was conquered by the Roman Empire by 43A.D. During the course of their 400 year rule of the Celts, the Romans combined their own festivals of Feralia (commemorated the passing of the dead) and Pomona (goddess of fruit and trees) with the Celtic celebration of Samhain. Pope Gregory III would go on to expand the festival to include the observance of All Saints Day on November 1st. As Christianity expanded in Ireland, the church made November 2nd All Souls Day to honor the dead. All Saints Day was also called All-Hallows, so the night before (October 31st) began to be known as All-Hallows Eve and eventually Halloween.