Halloween may be a secular affair today, dominated by candy, costumes and trick-or-treating, but the holiday is rooted in an annual Celtic pagan festival called Samhain (pronounced "SAH- wane") that was then appropriated by the early Catholic Church some 1,200 years ago.

The ancient Celts were an assortment of tribes and small kingdoms once scattered across western and Central Europe with distinctive languages and culture, explains Frederick Suppe, a historian specializing in Celtic and medieval history at Ball State University in Indiana.

Even after the Romans conquered their realm, Celts continued to survive and thrive in places such as Brittany, Cornwall, Ireland, the Isle of Man, Scotland, and Wales.

Samhain, the Celtic festival that is the ancestor of Halloween, was related to the Celts’ way of looking at the world. “All the Celtic peoples conceived of a fundamental dichotomy between light and dark, with the former representing positive, lucky, fruitful values and the latter representing negative, threatening, destructive values,” Suppe explains.

The Celtic year began at sundown at the end of the autumn harvest, continued through the darkness of winter and the early spring into the brightness of the summer growing season, and concluded with the harvest. Two big holidays divided their year—Beltane, which took place April 30-May 1 on our calendar, and Samhain, which occurred from October 31 to November 1.

Samhain was the moment when the spiritual world became visible to humans, and the gods enjoyed playing tricks on mortals. It was also a time when the spirits of the dead mingled with the living.

The Celts believed in gathering all their harvests by Samhain, “so that it would not be damaged by the evil or mischievous spirits who could return on the first evening of the dark half of the year,” Suppe says. “Token offerings of the harvested food should be offered to the spirits to placate them.”

The importance of pre-Christian customs to people’s lives apparently wasn’t lost upon the early Catholic Church. Pope Gregory I, also known as St. Gregory the Great, who headed the Church from A.D. 590 to 604, advised a missionary going to England that instead of trying to do away with the religious customs of non-Christian peoples, they simply should convert them to a Christian religious purpose. For example, “the site of a pagan temple could be converted to become a Christian church,” Suppe says.

In that fashion, Samhain, the Celts’ dark supernatural festival, eventually was converted and given a Christian context.

“The ancient Celts believed that all sorts of threatening spirits were out and about on Samhain,” Suppe says. “The early medieval Christian church believed in saints—Christians who were remarkable for their devout religious beliefs and lives.” But saints also had a supernatural side, such as their involvement in miraculous occurrences.

So the Church mixed the traditions involving Celtic spirits and Catholic saints. In the 800s, the Church designated November 1 as All Saints’ Day.

“The old beliefs associated with Samhain never died out entirely,” folklorist Jack Santino wrote in a 1982 article for the American Folklife Center. “The powerful symbolism of the traveling dead was too strong, and perhaps too basic to the human psyche, to be satisfied with the new, more abstract Catholic feast honoring saints.”

Instead, the first night of Samhain, October 31, became All Hallows Day Evening, the night before the saints were venerated. That name eventually morphed into Halloween, and it became the time when Christians could turn the supernatural symbolism and rituals of Samhain into spooky fun.

One of the rituals adopted from the Celts was pumpkin carving, which held religious significance. “The jack-o-lantern custom consists of placing fire—which imitates the good magic of the sun—inside a hollowed out vegetable, representing the harvest,” Suppe says. It was done “in hopes that the good magic will help to preserve the harvested food through the dark half of the year, until the next growing season could replenish the community's food stocks.”

Later, in Ireland and Scotland, people developed the custom of using similarly-carved vegetable lanterns to scare off the mythical character of Stingy Jack, who wandered the Earth because the devil wouldn’t let him into hell.

Similarly, “the practice of trick-or-treating originates in the Celtic custom of giving token bits of the harvest to spirits wandering outside of houses on the evening of Samhain, to placate them and prevent them from doing destructive things to the harvest or to homes,” Suppe says. Once Christianity became established in the Celtic regions, young, unmarried men would parade around on Halloween, going to houses and calling for gifts to the spirits.

“This was a time when the hard work of the harvest was done, so they could indulge in some pranks to let off steam,” Suppe says. In Scotland, the groups of young men were called “guisers” (pronounced “GEE-zers”) because they wore disguises—the beginning of the custom of wearing Halloween costumes.

Centuries later, Halloween customs were brought to the United States by immigrants from Ireland, Scotland and other ancient homelands of the Celts. As an 1894 article in Christian Work describes the holiday: Halloween is a night "when witches, evil spirits, and all mischief brewing sprites went forth on dark and mysterious midnight revels.”