Halloween originated 2000 years ago among Celtic pagans in Ireland, England, and France. They celebrated Samhain on October 31st, when spirits of the dead were believed to return to Earth. People wore costumes to avoid being recognized as spirits. As Christianity spread, the holiday was changed to All Saints' Day on November 1st. Over time, traditions like wearing costumes and trick-or-treating developed. Irish immigrants brought Halloween to America in the 1800s, where it became popular among children going door-to-door for candy using pumpkins as jack-o-lanterns.