Halloween is usually celebrated on October 31, but its origins are actually in the ancient Celtic festival of Samhain. According to folklorists, our modern-day tradition of dressing up in costumes for trick or treating can be traced back 2,000 years to a holiday that would become known as Samhain. Originally practitioners would light bonfires and dress up in costumes to ward off ghosts during the ancient Celtic festival of Samhain! Over the years, Halloween developed into an event where people trick-or-treat, carve jack-o'-lanterns, attend parties, dress in costumes, and eat treats.