Samhain — Old Irish, pronounced approximately "SAH-win," meaning possibly "summer's end" or "assembly in November" — is documented extensively in medieval Irish literature as the most important festival of the year. The Tochmarc Emire (The Wooing of Emer), the Cath Maige Tuired (The Second Battle of Mag Tuired), and numerous other medieval texts describe Samhain as a time of cosmic significance — when the síde (fairy mounds) opened, the Tuatha Dé Danann walked freely in the human world, and the dead returned to visit the living.
The festival marked the end of the pastoral year — the cattle were brought down from the summer pastures, the weakest slaughtered for the winter meat supply, and the great assembly at Tara was held. It was simultaneously the third harvest (the harvest of meat and preserved food), the Celtic new year (the year began in darkness rather than light) and the night of maximum supernatural danger and opportunity.
Crucially, Samhain was a liminal time — a period outside ordinary time, when the usual rules were suspended. The world was between summer and winter, between the old year and the new, between the living and the dead. At such thresholds, extraordinary things became possible. The great mythological events of Irish literature — the first battle of Mag Tuired, the conception of Cú Chulainn, the founding of the Dagda's kingship — frequently occur at Samhain, because Samhain is when the world is most open to transformation.
The fire was extinguished in every household on Samhain eve and relit from the great communal fire on the Hill of Tlachtga — each family carrying home a flame from the central sacred fire, reconnecting the domestic hearth to the cosmic source. This annual renewal of fire symbolised the renewal of the community itself: the old year ended in darkness; the new year began with shared light.