The origin of Halloween dates back to the ancient Celtic festival of Samhain which took place on the night of October 31st. Celts believed that on that night the boundary between the worlds of the living and the dead became blurred. To commemorate the event, they built huge bonfires, where people gathered and wore costumes, typically of animal heads and skins, and told fortunes.
Later under Romans rule, Roman festivals were combined with traditional Celtic celebrations. Again, celebrations were combined under the influence of Christianity. The church designated November 1st All Saints’ Day, a time to honor saints and martyrs and November 2nd All Souls’ Day, a day to honor the dead. All Souls’ Day was celebrated similarly to Samhain, with big bonfires, parades, and dressing up in costumes as saints, angels, and devils. Together, the celebrations were called Hallowmas.
As European immigrants came to America, they brought their varied Halloween customs with them. However celebration of Halloween in colonial times was extremely limited.
In the second half of the nineteenth century, America was flooded with a new wave of immigrants. They helped to popularize the celebration of Halloween nationally. Americans began to dress up in costumes and go house to house asking for food or money, a practice that eventually became today’s “trick-or-treat”. Trick-or-treating probably dates back to the early All Souls’ Day parades in England. During the festivities, poor citizens would beg for food and families would give them pastries called “soul cakes” in return for their promise to pray for the family’s dead relatives.
The distribution of soul cakes was encouraged by the church as a way to replace the ancient practice of leaving food and wine for roaming spirits. The practice, which was referred to as “going a-souling” was eventually taken up by children who would visit the houses in their neighborhood and be given ale, food, and money.
At the turn of the century, Halloween parties for both children and adults became the most common way to celebrate the day. By the 1920s and 1930s, Halloween had become a secular holiday, with parades and town-wide parties. By the 1950s Trick-or-treating was a major part of the holiday.
Today Halloween is now the second largest commercial Holiday in the united States with Americans spending a near $6.9 million annually on the holiday!
Article taken from the History Channel, http://www.history.com/content/halloween