The origins of Halloween date back to Celtic paganism and the feast of Samhain in the British Isles, which Celts believed was when spirits came into the world. The Celts put out treats to placate the spirits for another year. Halloween also has aspects of a Roman harvest feast that celebrated the goddess Pomona.
Halloween evolved once Christianity reached the Celtic people. The new religion fused the pagan culture with Christian practices. Because Christianity's All Souls' Day was at the same time as the pagan feast of Samhain, the two were merged into All Hallows' Eve.
Eventually Irish immigrants brought Halloween to America. Their practices became integrated into American culture. Slowly Halloween evolved into a holiday focusing on children, costumes and trick-or-treating. A huge part of this evolution occurred because of the money that could be made off the holiday, according to scholar Regina Hansen. Halloween is often considered an American holiday, because Halloween assumed its popular form in the United States.
Halloween is celebrated in other countries, particularly in England, Wales, Scotland and Ireland, much as it is celebrated in the United States. However, England also celebrates Guy Fawkes Day, which falls on Nov. 5, in conjunction with Halloween, by lighting off fireworks.