Samhain ( /ˈsɑːwɪn/, /ˈsaʊ.ɪn/, or /ˈsaʊn/) is a Gaelic festival marking the end of the harvest season and the beginning of winter or the "darker half" of the year. Most commonly it is held on 31 October–1 November, or halfway between the autumn equinox and the winter solstice. Along with Imbolc, Beltane and Lughnasadh it makes up the four Gaelic seasonal festivals. It was observed in Ireland, Scotland and the Isle of Man. Kindred festivals were held at the same time of year in other Celtic lands; for example the Brythonic Calan Gaeaf (in Wales), Kalan Gwav (in Cornwall) and Kalan Goañv (in Brittany).
Samhain is mentioned in some of the earliest Irish literature. Many important events in Irish mythology happen or begin on Samhain. It was the time when cattle were brought back down from the summer pastures and when livestock were slaughtered for the winter. In much of the Gaelic world, bonfires were lit and there were rituals involving them, as at Beltane. People and their livestock would often walk between two bonfires as a cleansing ritual, and the bones of slaughtered livestock were cast into its flames. Samhain (like Beltane) was seen as a time when the "door" to the Otherworld opened enough for the souls of the dead, and other beings, to come into our world. Feasts were had, at which the souls of dead kin were beckoned to attend and a place set at the table for them. It has thus been likened to a festival of the dead. People also took steps to protect themselves from harmful spirits, which is thought to have led to the custom of guising. Divination was also done at Samhain.
It was popularized as the "Celtic New Year" from the late 19th century, following Sir John Rhys and Sir James Frazer. It has been linked with All Saints' Day (and later All Souls' Day) since the 9th century, when the date of that holiday was shifted to November 1. Both have strongly influenced the secular customs of Halloween.
Samhain is still celebrated as a cultural festival by some (though it has mostly been replaced by Halloween) and, since the 20th century, has been celebrated as a religious festival by Celtic neopagans and Wiccans. Neopagans in the Southern Hemisphere often celebrate Samhain at the other end of the year (~30 April–1 May).