Halloween originated from the ancient Celtic festival of Samhain, marking the end of the harvest season and the beginning of winter. People carved scary faces into turnips and pumpkins to ward off spirits. It was believed that on Halloween, the veil between the living and dead was thin, allowing souls to return to Earth. To avoid spirits, houses were decorated with bones and skulls. People wore costumes to disguise themselves from ghosts. Food was left outside homes to appease ghosts and prevent them from entering, which is the origin of modern trick-or-treating.