Halloween, and its associated costume tradition, originates from the ancient Celtic festival of Samhain, an annual New Year’s Eve party of sorts, celebrated in what is now the United Kingdom and northern France. The Celts celebrated the new year on November 1 and believed that the holiday was the one night of the year that the spirits of deceased loved ones would return (much like the holiday now known as All Saints Day). The Celts believed deities would play tricks on humans the night of the festival. So at sunset on October 31, the night before the new year, they celebrated by lighting bonfires and dressing up as animals to ward off evil spirits. This might be the original reason why trick-or-treaters march around neighborhoods in furry cat ears and DIY ladybug costumes asking for candy. While Samhain is still celebrated in some areas of Ireland and Scotland, much of the world has split this ancient celebration into two separate holidays: A day set aside to honor loved ones who have passed on (All Saints Day, celebrated on November 1) and a holiday celebrated with costumes, candy, and carved pumpkins (Halloween, celebrated on October 31). So whether you’re putting together a DIY elephant costume or purchasing a costume from Target this year, you’ll be participating in a centuries-old Halloween tradition.