Samhain (ˈsɑːwɪn , ˈsaʊɪn , ˈsɣəunj, ˈs̪ãũ.ɪɲ; Sauin ˈsoːɪnj) is a Gaelic festival on 1 November marking the end of the harvest season and beginning of winter or "darker half" of the year. Celebrations begin on the evening of 31 October, since the Celtic day began and ended at sunset. This is about halfway between the autumnal equinox and winter solstice. It is one of the four Gaelic seasonal festivals along with Imbolc, Beltaine, and Lughnasa. Historically it was widely observed throughout Ireland, Scotland, Galicia, and the Isle of Man (where it is spelled Sauin). A similar festival was held by the Brittonic Celtic people, called Calan Gaeaf in Wales, Kalan Gwav in Cornwall, and Kalan Goañv in Brittany.
Samhain is believed to have Celtic pagan origins and some Neolithic passage tombs in Ireland are aligned with the sunrise at the time of Samhain. It is first mentioned in the earliest Irish literature, from the 9th century, and is associated with many important events in Irish mythology. The early literature says Samhain was marked by great gatherings and feasts and was when the ancient burial mounds were open, which were seen as portals to the Otherworld. Some of the literature also associates Samhain with bonfires and sacrifices.
The festival was not recorded in detail until the early modern era. It was when cattle were brought down from the summer pastures and when livestock were slaughtered. As at Beltaine, special bonfires were lit. These were deemed to have protective and cleansing powers and there were rituals involving them. Like Beltaine, Samhain was a liminal or threshold festival, when the boundary between this world and the Otherworld thinned, meaning the aos sí (the 'spirits' or 'fairies') could more easily come into our world. Most scholars see the aos sí as remnants of pagan gods. At Samhain, they were appeased with offerings of food and drink, to ensure the people and their livestock survived the winter. The souls of dead kin were also thought to revisit their homes seeking hospitality, and a place was set at the table for them during a meal.