Halloween - October 31st - began as the Celtic New Years celebration, and was the most important of their four "quarter days". Popular belief once held that on this last night of the old year, the Celtic "Lord of Death" (named "Samhain") gathered together and judged the souls of all those who had died in the passing year However, we now recognize that the Celts had no "Lord of Death", and Samhain (pronounced "sah-wen") means "summer's end", and was the name for the festival celebrated at the end of October. On this night all fires were extinguished, and the Druids (the Celts' priests) kindled a new fire, which was distributed the next morning to each household. As the boundary between two years, Samhain was a time for not only for paying off debts, but also for fortune-telling and visits by spirits (the sidhe of Celtic mythology).
Another popular misconception about the history of Halloween is that it also stems from a Roman harvest festival honoring Pomona, "the goddess of fruits". In fact, Pomona was a minor figure in Roman mythology who did not merit her own holiday. However, the Roman celebrations of Feralia (held in February) and Lemuria (held in May) were both festivals of the dead, and may have influenced the evolution of Halloween (especially Lemuria, a three-day event which ended on May 13, the original date set by Pope Boniface IV for the celebration of All Saints' Day).
The Catholic celebration of All Saints' Day was officially inaugurated in 609 A.D., although the Christian influence on Halloween actually begins in 601 A.D., when Pope Gregory I instructed his missionaries that, rather than obliterate native peoples' customs and beliefs, they should try to use them; hence, Catholic holy days were set at the times of native holy days, celebrations and festivals. As Christian missionaries moved into Ireland, they practiced Gregory's doctrine of "syncretism" and replaced the Celts' Samhain with All Saints' Day (Pope Gregory III moved the observation to November 1 in the eight century). In 998 A.D. St. Odilo, Bishop of Cluny, instituted a day of prayer and special masses for the souls of the dead. This day became All Souls' Day, and was set for November 2nd. All Souls' Day may have given Halloween some of its gloomier aspects.
Another day with connections to Halloween is the British Guy Fawkes Day, celebrated on November 5. Guy Fawkes was a Roman Catholic who planned to blow up the Protestant House of Parliament on November 5, 1606 (his real goal was the assassination of King James); luckily for his intended victims, he was apprehended and executed. Afterwards, the anniversary of the day was celebrated by building straw effigies, entreating passersby for "a penny for the Guy", and finally burning "the Guys" in bonfires. British colonists to America brought the tradition with them, although it has since died out here. It wasn't until the later influx of Irish and Scottish immigrants to America that Halloween began to evolve into the celebration we know today (remember, the Scots/Irish are the descendents of the Celts).
Throughout the 19th and early 20th centuries, Halloween was a time of fortune-telling, harvest celebration and prank-playing; it wasn't until the 1930's that the modern tradition of trick-or-treat came into practice (although it took the next two decades for the practice to spread across the country). Trick-or-treat is descended from a number of different mumming practices, which involve begging food in exchange for a costumed performance. In England (as late as the 19th century), the poor in some areas of the country went about singing for "soul-cakes" or money, promising to pray and to spend the alms in masses for the dead; in America, a "masked solicitation" ritual centered on Thanksgiving died out just before trick-or-treat was born. And in some areas of Canada and America, a Christmas ritual called "belsnickling" involved costumed performers going house-to-house and demanding that spectators guess their identities. Thus, trick-or-treat may derive from a combination of Guy Fawkes Day, Thanksgiving, Christmas, and All Souls' Day traditions.
Halloween has undergone several curious turns over the last century. In the 19th century and early 20th century, Halloween was primarily the province of adults, with some older children participating in pranks (sometimes so elaborate as dismantling an entire wagon and rebuilding it on top of a barn roof!). However, from the 1930's to the 1970's, Halloween was primarily thought of as a night for children. Folklorist Jack Santino has suggested that Halloween was about inverting societal norms, so that children held power over adults for one night a year. However, around the 1960's, things began to change: Trick-or-treating was menaced by the suggestion that disturbed individuals were poisoning candy and putting razor blades in apples, while adult costuming and parties gained in popularity. Today, Halloween seems to have once more been largely claimed by adults as their own, with the haunted attractions industry now claiming billion-dollar-a-year revenues; meanwhile, carefully supervised activities for children have replaced the festival nature of trick-or-treating.
Halloween is one of our oldest and most fascinating holidays, and its endurance through the millennia indicates that it speaks to a part of our psyche so deep that it will doubtless exist in some form as long as humankind still passes from fall to winter.