Which spooky festival is traditionally observed on the 31st of October?

Halloween, observed on October 31st, has roots stretching back to ancient Celtic traditions. The Celts believed that on this night, the boundary between the worlds of the living and the dead blurred. They celebrated Samhain, a festival marking the end of summer and the harvest, and a time when spirits could roam freely. Over time, as Christianity spread, Samhain evolved into All Hallows' Eve, the night before All Saints' Day. Customs like dressing up in costumes and lighting bonfires were meant to ward off evil spirits. The tradition of trick-or-treating developed later, with people going door-to-door offering prayers for the dead in exchange for food. So, Halloween's spooky atmosphere and traditions of costumes, pumpkins, and trick-or-treating make it the only correct answer among the choices, as it's the only festival traditionally observed on October 31st with these characteristics.