Halloween traces its origins to the ancient Celtic festival of Samhain, celebrated over 2,000 years ago in the area that is now Ireland, the United Kingdom, and northern France. The Celts, whose new year began on November 1, believed that on the night before, the boundary between the worlds of the living and the dead became blurred. On the evening of October 31, they celebrated Samhain, when it was believed that the ghosts of the dead returned to earth. Celts thought the presence of these otherworldly spirits made it easier for their priests, the Druids, to make predictions about the future.

By 43 AD, the Roman Empire had conquered the majority of Celtic territory. During the 400 years they ruled the Celtic lands, two Roman festivals were combined with Samhain: Feralia, a day in late October when Romans commemorated the passing of the dead, and a day to honor Pomona, the Roman goddess of fruit and trees. The apple likely became associated with Halloween through Pomona, and bobbing for apples remains a popular Halloween party game to this day.

In the seventh century, Pope Boniface IV designated November 1 as All Saints' Day, a time to honor saints and martyrs. The celebration was also called All-hallows, and the night before, the traditional night of Samhain in Celtic religion, began to be called All-Hallows Eve, eventually shortened to Halloween. In 1000 AD, the church made November 2 All Souls' Day, a day to honor the dead, creating a three-day observance known as Allhallowtide.

In the United States and Canada, trick-or-treating is the defining Halloween tradition. Children dress in costumes and go door to door collecting candy, a practice that evolved from the medieval custom of "souling," where the poor would visit homes on All Souls' Day and receive food in return for prayers for the dead. Americans spend over $3.6 billion on Halloween candy annually, making it the second-largest commercial holiday in the country after Christmas.

In Mexico and much of Latin America, Dia de los Muertos (Day of the Dead) is celebrated from October 31 through November 2. Families build ofrendas, colorful altars adorned with marigolds, candles, photographs, and the favorite foods of deceased loved ones. Sugar skulls (calaveras de azucar) are crafted as both offerings and decorations, and cempasuchil (marigold) petals are scattered in paths to guide spirits home. UNESCO recognized Dia de los Muertos as an Intangible Cultural Heritage of Humanity in 2008.

In Ireland, where Halloween originated, the holiday is still widely celebrated with bonfires, games, and traditional foods. Barmbrack, a type of fruitcake, is baked with hidden objects inside: a ring predicts marriage, a coin means wealth, and a thimble foretells spinsterhood. In the Philippines, Pangangaluluwa is practiced, where children go door to door singing and asking for prayers for those in purgatory. In Japan, Halloween has grown into one of the biggest street festivals of the year, with the Shibuya district in Tokyo hosting massive costumed gatherings every October.