Samhain, pronounced SAH-win, (Irish for ‘Summer’s End’) is a Gaelic festival marking the end of the harvest season and the beginning of winter or the “darker half” of the year. Traditionally, it is celebrated from 31 October to 1 November, as the Celtic day began and ended at sunset. Samhain is about halfway between the autumn equinox and the winter solstice. At this time of the year, the veil between the worlds is thin. Like indigenous shamans all over the world, Celtic druids and seers were believed to enter the darkness to contact the spirits who dwelled in the hidden realms and brought back wisdom for their people.
Two hills in the Boyne Valley were associated with Samhain in Celtic Ireland, Tlachtga and Tara. Tlachtga was the location of the Great Fire Festival which begun on the eve of Samhain (Halloween). Tara was also associated with Samhain, however it was secondary to Tlachtga in this respect. The entrance passage to the Mound of the Hostages on the Hill of Tara is aligned with the rising sun around Samhain. The Mound of the Hostages is 4,500 to 5000 years old, suggesting that Samhain was celebrated long before the first Celts arrived in Ireland about 2,500 years ago.
According to Wikipedia, Samhain is mentioned in some of the earliest Irish literature and 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. Samhain was seen as a liminal time, when the boundary between this world and the Otherworld could more easily be crossed. This meant the ‘spirits’ or ‘fairies’, could more easily come into our world. It was believed that the fairies needed to be appeased to ensure that the people and their livestock survived the winter. Offerings of food and drink were left outside for them. The souls of the dead were also thought to revisit their homes seeking hospitality. 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, and disguising oneself from, the fairies.
In the 9th century, Western Christianity 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. Historians have used the name ‘Samhain’ to refer to Gaelic ‘Halloween’ customs up until the 19th century.