Ancient Celts, who lived 2,000 years ago in the area that is now Ireland, the United Kingdom and northern France, marked Samhain at the midpoint between the fall equinox and the winter solstice. During this time of year, hearth fires in family homes were left to burn out while the harvest was gathered. After the harvest work was complete, celebrants joined with Druid priests to light massive bonfires and pray.

Celts believed that the barrier between the physical and spirit worlds was breachable during Samhain. It was expected that ancestors might cross over during this time as well, and Celts would dress as animals and monsters so that fairies were not tempted to kidnap them.

In the 7th century, the Catholic Church established November 1 as All Saints' Day, a day commemorating all the saints of the church. By the 9th century, the influence of Christianity had spread into Celtic lands, where it gradually blended with and supplanted older Celtic rites. In 1000 A.D., the church made November 2 All Souls’ Day, a day to honor the dead. It’s widely believed today that the church was attempting to replace the Celtic festival of the dead with a related, church-sanctioned holiday.

The All Saints’ Day celebration was also called All-hallows or All-hallowmas (from Middle English Alholowmesse meaning All Saints’ Day) and the night before it, the traditional night of Samhain in the Celtic religion, began to be called All-Hallows Eve and, eventually, Halloween. Over many centuries, the three holidays—All Saints’ Day, All Souls' Day and Samhain—essentially merged into one: Halloween. (The Catholic Church still recognizes All Saints’ Day and All Souls' Day today, and some Wiccans and Celtic Reconstructionists commemorate Samhain.)

In England and Ireland during All Saints' Day and All Souls' Day celebrations, poor people would visit the houses of wealthier families and receive pastries called soul cakes in exchange for a promise to pray for the souls of the homeowners’ dead relatives. Known as "souling," the practice was later taken up by children, who would go from door to door asking for gifts such as food, money and ale—an early form of trick-or-treating.

The practice of carving faces into vegetables became associated with Halloween in Ireland and Scotland around the 1800s. Jack-o-lanterns originated from an Irish myth about a man nicknamed “Stingy Jack,” who tricked the Devil and was forced to roam the earth with only a burning coal in a turnip to light his way. People began to make their own versions of Jack’s lanterns by carving scary faces into turnips or potatoes and placing them into windows or near doors to frighten away Stingy Jack and other wandering evil spirits.

With the exception of Catholic-dominated Maryland and some other southern colonies, Halloween celebrations were extremely limited in early America, which was largely Protestant. It wasn't until the mid-19th century that new immigrants—especially the millions of Irish fleeing the Irish Potato Famine—helped popularize the celebration nationally.

These immigrants celebrated as they did back in their homelands—especially by pulling pranks. In the late 1800s, common Halloween tricks included placing farmers’ wagons and livestock on barn roofs, uprooting vegetables in backyard gardens and tipping over outhouses. By the early 20th century, vandalism, physical assaults and sporadic acts of violence were not uncommon on Halloween.

Haunted or spooky public attractions already had some precedent in Europe. Starting in the 1800s, Marie Tussaud’s wax museum in London featured a “Chamber of Horrors” with decapitated figures from the French Revolution. In 1915, a British amusement ride manufacturer created an early haunted house, complete with dim lights, shaking floors and demonic screams.

In the U.S., the Great Depression kickstarted the trend. By then, violence around Halloween—no doubt exacerbated by the dire economic conditions—had reached new highs. Parents, concerned about their children running amok on All Hallows' Eve, organized “haunted houses” or “trails” to keep them off the streets.

Costumes and disguises have figured into Halloween celebrations since the holiday's earliest days. But it wasn't until the mid-20th century that costumes started to look like what we know them as today.

Around the same time neighborhoods began organizing activities such as haunted houses to keep kids safe and occupied, costumes became more important (and less abstract and scary). They began to take the form of things children would have seen and enjoyed, like characters from popular radio shows, comics and movies. In the 1950s, mass-produced box costumes became more affordable, so more kids began to use them to dress up as princesses, mummies, clowns or more specific characters like Batman and Frankenstein’s monster.

While in general the fears about poisoned Halloween candy have been overblown, crimes involving poison have occurred. The most infamous case took place on October 31, 1974. That’s when a Texas man named Ronald O’Bryan gave cyanide-laced pixie sticks to five children, including his son. The other children never ate the candy, but his eight-year-old son, Timothy, did—and died soon after.

The paranoia reached new heights in the early 1980s after a rash of Tylenol poisonings in which cyanide-laced acetaminophen was placed on store shelves and sold. After the Tylenol murders, which are still unsolved, warnings about adulterated Halloween candy increased.