Some attribute the wearing of Halloween costumes to the Gaelic festival of Samhain (or Samuin), a Celtic festival marking the end of the harvest season and the beginning of winter in Gaelic Ireland or the “darker half” of the year. It is celebrated from sunset on 31 October to sunset on 1 November, or about halfway between the autumn equinox and the winter solstice.
Samhain was seen as a time, when the spirits or fairies (the aos sí) could more easily come into our world. It was believed that the Aos Sí needed to be propitiated to ensure that the people and their livestock survived the winter. Offerings of food and drink were left for them. The souls of the dead were also thought to revisit their homes. Feasts were had, at which the souls of dead kin were beckoned to attend and a place set at the table for them. Mumming and guising were part of the festival, and involved people going door-to-door in costume (or in disguise), often reciting verses in exchange for food. The costumes may have been a way of imitating, or disguising oneself from the Aos Sí. Divination rituals were also a big part of the festival and often involved nuts and apples.
In the 9th century, the Roman Catholic Church shifted the date of All Saints’ Day to 1 November, while 2 November later became All Souls’ Day. Over time, Samhain and All Saints’/All Souls’ merged to create the modern Halloween.