Halloween's origins date back to the ancient Celtic festival of Samhain (pronounced sow-in). The Celts celebrated their new year on November 1 at the end of the harvest and the beginning of the dark, cold winter, a time that often meant an increased death toll due to harsh weather. Celtic traditions held that on the night before the new year, the boundary between the world of the living and the world of the dead was easily crossed, and that both living people and spirits of the dead could cross over. Some of these spirits were helpful, and aided the Celts in predicting the future. Other spirits were malicious and harmful.

In order to appease all the spirits, huge bonfires were made and crops and animals were burned as sacrifices to the undead. These ritualistic sacrifices were traditionally done wearing costumes made from the heads and skins of animals.

Romans also had similar holidays in the fall. In late October, Romans commemorated the dead with a holiday called Feralia, This was followed with another holiday to honor Pomona, the Roman goddess of fruit and trees. The symbol of Pomona is the apple and the incorporation of this celebration into existing Celtic and eventual Christian holidays is likely the root of the tradition of bobbing for apples.

As Christianity spread across Europe, Pope Bonifice IV declared All Saints Day as a day to honor all saints and martyrs. This was also called All-Hallows or All-Hallowsmas, which eventually became Halloween.

Trick-or-Treating dates back to the early All Souls' Day parades in England. During the parade, poor citizens would beg wealthier citizens for food and would be given "soul cakes" in return for their promise to pray for the wealthy family's dead relatives.

The practice, which was referred to as "going a-souling" eventually became the “job” of poor children who would visit the houses in their neighborhood and be given ale, food, and money.

Many of the popular symbols of Halloween, as it’s celebrated in the United States, have their roots in these Celtic and early Christian traditions.

The wearing of costumes, for instance, has roots in European traditions and the Celtic tradition of appeasing the Gods. On Halloween (or All Hallows Eve as it was known) people were afraid to leave the house, fearing they may be abducted by the ghosts of people they knew. In order to avoid recognition, they wore costumes, if they left the house at all. And in order to stop the ghosts from entering their houses, they would leave out bowls of food and money, hoping that was enough to appease them and send them to the next house.