Seven out of 10 Americans will be celebrating Halloween this Saturday, spending just under $7 billion on candy, decorations, costumes and greeting cards, with $350 million of this bonanza going toward the purchase of pet costumes, according to a survey by the National Federation of Retailers.

Halloween has gone big-time in the U.S., but where exactly does the holiday hail from, and why is there such a strong supernatural element to it? Some argue that Halloween's origins stretch all the way back to ancient Rome; others maintain that its date and basic memes are Christianized versions of pagan rites.

Although opinions vary widely, UCLA boasts two preeminent experts on mythology and folklore who have studied the holiday's history for years: medieval scholar and distinguished research professor of English Henry Kelly and Joseph Nagy, professor of Celtic folklore and mythology.

Kelly notes that many Halloween history hunters accept the mistaken contention put forth in 1890 by famed Scottish social anthropologist Sir James G. Frazer that Oct. 31 was the Celtic pagan festival of death. Long-ago Celtic traditions do appear to echo throughout the Halloween canon. But there is a difference between influence and origin.

Halloween, Kelly asserts, "is not a pagan celebration. And despite concern about its spookier elements, there's nothing diabolical about the way Halloween is celebrated, adds the emeritus professor, who, as the author of “Satan: A Biography,” certainly qualifies as an authority on the subject of the devil.

In fact, the night of Oct. 31 is the evening before the Christian festival of All Hallows' [Saints'] Day on Nov. 1, and our Halloween took its form in particular from the way in which the night was celebrated in Celtic lands like Ireland, Scotland and Wales. The word "Halloween" is a Scottish variation of All Hallows' Even (or evening) and, according to the Oxford English Dictionary, was first used in Old English as far back as the 16th century.

"In the second half of the first millennium A.D., as the cult of the saints grew in western Christendom, the church established Nov. 1 as the day on which to celebrate all the saints, especially those who did not have their own feast days," Nagy explains. "As is true with all church feast days, the observance of All Saints' Day — which has historically involved fasting as well as special prayers and rituals — begins on the evening of the preceding day."

The church subsequently established Nov. 2 as All Souls' Day, which Nagy explains, was "a time to commemorate and pray for those who had died in general, particularly those who might still be experiencing a process of purification before they could be admitted into heaven. ... While the notion of the souls of the dead surviving as ghosts was hardly sanctioned by the church, the doctrinal concept of purgatory, an intermediary stage between earthly existence and heavenly afterlife, added impetus to long-lived popular beliefs in and stories about the restless 'undead.'"

In the U.S., "There was no Halloween celebration or even any idea of Halloween until the 19th century, when it came with the Irish immigrants," explains Kelly. "They brought two features: vandalism, unfortunately, and parties. There used to be a lot of fortune-telling parties and [apple] dunking and the like."

Still, almost from the moment it became part of our national consciousness, the eve of All Saints' Day drew media attention. Harper's Magazine published the article "A Legend of All-Hallow Eve" in November of 1879 — but by the middle of the 20th century, Halloween had become mostly a children's holiday, shorn of religious references and emphasizing fun and fantasy.

So where did Halloween's otherworldliness really come from? Again, accounts vary, and nobody knows for certain.

The Oxford English Dictionary, for example, notes that "The last evening of October was 'old-year's night,' the night of all the witches, which the church transformed into the Eve of All Saints."

Kelly asserts that "the idea of Halloween first significantly hit upon the mind of the English-speaking world, I would say, in 1786, when [Scottish poet] Robert Burns published his poem 'Halloween,' [in which he explained] what the custom of Halloween was in western Scotland. It was thought that there were witches and devils and fairies abroad on that day, and people had a lot of parties. And his poem is all about these parties, which were basically for courting couples. For instance, they'd go out in the backyard and pull out a cabbage and knock off the dirt and look at the root structure, and that would give them some indication of what the couple would be like in 50 years."

Medieval matchmaking aside, Nagy considers that some elements of Celtic tradition do parallel this singular aspect of Halloween. He observes that "the eve of Nov. 1 — or 'Samhain' as it is known in Irish … or 'Caland Gaeaf,' the first day of winter in Welsh — was considered to be a temporal 'space' in which humans were more likely to run the risk of encounters with supernatural beings, more active at this time than they are through most of the rest of the year."

Nagy adds, "In-between times — i.e., times of transition — characteristically become infused with a sense of the supernatural, ranging from the religiously defined (e.g., saints, all souls) to the disquietingly undefinable (things that go bump in the night)."

So where did the Halloween traditions of dressing in costume and going door-to-door demanding treats and threatening tricks come from?

Again, local custom may have come into play here, Nagy surmises, because "fall, particularly November, was when especially younger adult members of the community would return from the activity of herding animals in upland pastures or from agricultural labor undertaken away from home … so this was a time when younger members of the community, having temporarily become strangers to it, would return home, often participating under the influence of tradition in performative rituals, traveling from house-to-house in costume, not always with benign intent."

The practice of dressing up and going door-to-door for food or money was commonplace in general during holidays in medieval Europe. In the U.S., however, both costumes and trick-or-treating have been part of Halloween beginning in the first half of the 20th century.

"The first reference to it occurs in a magazine in 1939,” Kelly says, “when the author talks about the 'age-old' salutation 'trick or treat' — but this is the first recorded instance of it. It was quite clearly a device to defuse the vandalism. And it didn't exactly work, because people [continued to] soap windows and TP trees and so forth."

But not so much anymore, which may be one reason why Halloween is more popular than ever in the U.S.

"Now it's just indulging in all sorts of fantasies," says Kelly. "It's just a joyful parade and celebration, a homegrown holiday, like Thanksgiving."

To Nagy, Halloween is "a remarkable demonstration of both the shelf life of folklore and of its adaptability to changing circumstances and new environments."

This has been condensed from a story in UCLA Magazine. Read the complete story here.