Halloween originated from the ancient Celtic festival of Samhain, celebrated on November 1st. It was a day to honor the dead and celebrate the end of the harvest season. When the Romans conquered Celtic lands, they combined their own autumn festival with Samhain, shifting it to November 1st. In the 19th century, Irish and Scottish immigrants brought Halloween traditions to North America. Today, Halloween is celebrated commercially with costumes, trick-or-treating, and carving jack-o'-lanterns, though some traditions stem from ancient Celtic beliefs about contacting the dead.