Halloween is an annual holiday celebrated each year on October 31, and Halloween 2019 occurs on Thursday, October 31. It originated with the ancient Celtic festival of Samhain, when people would light bonfires and wear costumes to ward off ghosts. In the eighth 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 sweet treats.

Halloween's origins date back to the ancient Celtic festival of Samhain. 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 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. 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. For a people entirely dependent on the volatile natural world, these prophesies 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 dieties. 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, the re-lit their hearth fires, which they had extinguished earlier that evening, from the sacred bonfire to help protect them during the coming winter. 

By 43 A.D., the Roman Empire had conquered  the majority of Celtic territory. In the course of the 400 years that they ruled the Celtic lands, 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 of honor to 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.

The celebration of Halloween was extremely limited in colonial New England because of the rigid Protestant belief systems there. Halloween was much more common in Maryland and the southern colonies.

As the beliefs and customs of different European ethnic groups and the American Indians meshed, a distinctly American version of Halloween began to emerge. The first celebration included "play parties", which were public events held to celebrate the harvest. Neighbors would share stories of the dead, tell each other's fortunes, dance and sing.

Colonial Halloween festivities also featured the telling of ghost stories and mischief-making of all kinds. By the middle of the nineteenth century, annual autumn festivities were common, but Halloween was not yet celebrated everywhere in the country.

In the second half of the nineteenth century, America was flooded with new immigrants. These new immigrants, especially the millions of Irish fleeing the Irish Potato Famine, helped to popularize the celebration of Halloween nationally.