The origins of Halloween date back to 500 BC to the ancient Celtic festival of Samhain. On this day, which marked the end of summer and beginning of winter, the Celts believed the border between the living world and spirit world became thin, allowing spirits to pass through. To avoid harm from spirits, the Celts would dress up in scary costumes and masks to disguise themselves as spirits. Modern Halloween celebrations originated from Celtic traditions and is now commonly celebrated in Britain, the US, and Canada due to Celtic descendants settling in those areas.