Halloween is known and loved today as a time to wear costumes, go door to door asking for candy, and watch monster movies. But the holiday's origins go back centuries to the enactment of All Saints' Day, a Christian holiday. Along the way, it has also picked up traditions from Samhain, a Celtic festival celebrating the start of winter.
The name "Halloween" began as "All Hallows Eve." This became "All Hallow E'en," leading to "Hallowe'en," or Halloween. It was the evening before All Hallows Day, which was later called All Saints' Day. (In this case, "hallows" meant "saints.")
All Saints' Day, a feast for all martyrs and saints, was celebrated on November 1st for the first time during the 8th century, but customs varied regarding its observance. This date was officially established for all Catholic churches in 837 by Pope Gregory IV.
Starting in the 10th century, this feast was the eve of All Souls' Day, which soon came to overshadow it.
Taking place on November 2, All Souls' Day was a day of prayer for the dead. It was believed that the prayers of those still living could comfort dead souls, or elevate them from Purgatory. The observances began the previous evening with prayers and the ringing of church bells.
When England moved from Catholicism to Protestantism, the All Souls' Day bell-ringing was prohibited and no official services were conducted. Individuals and groups continued to find ways of observing the day, perhaps out of a feeling of obligation to their dead loved ones; reports dating to the 16th century refer to people praying in the fields by the light of torches or bonfires.
Another observance involved "soul cakes." These (and alms) were given to the poor, in return for which the poor would offer a prayer for the dead. The poor and their children in some areas would go "souling," going to the homes of the wealthy and asking for soul cakes, fruit, and alms, a practice mentioned by Shakespeare in The Two Gentlemen of Verona. ("You have learned... to speak puling [whiningly], like a beggar at Hallowmas.")
November 1st also marks an ancient Celtic festival named Samhain (pronounced sah-win), or "summer's end." While little is known for certain about its original observances, it appears to have been a pagan calendar feast on the opposite side of the year from Beltane. (Some otherwise reputable sources claim that Samhain is the name for a Celtic god of the dead. This is unquestionably false.)
There has been much speculation about the relationship between the Halloween–All Saints–All Souls holidays and Samhain. Some believe that the Christian observances were deliberately moved to November in order to take over the pagan holiday. There is, however, no evidence for this. Others suggest that the pagan celebration may have gained its associations with the dead from the Christian holidays, but this is also speculation at best.
A more likely explanation may be that the turn of autumn—with the harvest finished, the days getting colder, the nights getting longer, and everybody getting ready to face the winter—naturally leads to thoughts of death and the unknowable. Much as many different cultures mark the start of spring with light-hearted holidays and celebrations of fertility and renewal, autumn may attract holidays in which people focus on the other side of the life cycle.
On the other hand, it hardly matters whether the Christian and pagan holidays were originally related to one another; the two have been intermingled in the popular imagination for a long time.
The modern observances of Halloween are more recent than one might expect. The holiday had a rebirth in North America between the late 19th and early 20th centuries, probably through an influx of Irish immigrants. They brought with them traditions that combined features of the Celtic and Christian holidays, and celebrated with feasting, divinations, and mischief making.
Jack-o'-lanterns and trick-or-treating in costume both became Halloween fixtures in North America, and have since been exported back to Europe.
For more on modern Halloween observances, see Halloween Traditions.
There has been a backlash against Halloween by several groups in recent years. Some Christians object to its allegedly pagan origins, or to what they see as its celebration of witches and other "evil powers." Some neo-pagans object to the alleged Christian takeover of their holiday, or to what they see as a distorted, negative view of witches and magic. And some simply don't think it's safe for children to go out after dark taking candy from strangers. (The last of those groups often proposes safer celebrations.)
Still, as long as there are cold autumn nights, a steady supply of candy corn, and radio stations to play "The Monster Mash," there seems no danger of Halloween going away.