Americans celebrate Halloween because it was brought to the United States by early settlers, specifically those from Ireland. Although small Halloween celebrations occurred in colonial times, the large influx of Irish immigrants in the 19th century contributed to the nationwide popularity of the holiday.
Halloween is derived from an ancient Celtic festival called Samhain. It was believed that the veils between the worlds were weaker so spirits and faeries were able to walk the earth. In the southwestern United States, the celebration may also have been influenced by the Mexican holiday Dia de los Muertos (Day of the Dead), which stems from ancient indigenous rituals and is a celebration that honors dead friends and family.