This Halloween timeline template depicts a worldwide celebration on October 31. It evolved from the ancient Celtic ritual of Samhain, a festival commemorating the change of seasons from light to dark (summer to winter). This usually happens around November 1. Traditionally, a bonfire would be lit, and costumes would be worn to ward off evil spirits, as the ancient Celts believed that the veil between the worlds of the living and the dead was at its thinnest at this time of year. Early Christian officials attempted to impose their holiday to discourage their converts from participating in non-Christian festivals. Pope Gregory III declared November 1 to be All Saints' Day.