Lughnasadh — Old Irish, "the assembly of Lugh" — is the first of the three harvest festivals in the Celtic calendar (followed by Mabon at the autumn equinox and Samhain at the year's end). It falls on 1 August, the astronomical midpoint between the summer solstice and the autumn equinox, when the grain crops were typically beginning to ripen in Ireland.
The festival is named for the god Lugh — the "shining one," the master of all skills — who, according to medieval Irish mythology, established the festival in honour of his foster mother Tailtiu, who died of exhaustion after clearing the plains of Ireland for agriculture. The great fair of Teltown (Tailtiu's place) in County Meath was one of the most important assemblies in early Irish history — combining athletic and martial games, legal disputes, the arrangement of marriages, trading and celebration. The Aonach Tailteann was held into the Christian period and was formally revived in 1924 as an early version of the Irish Olympics.
Medieval Irish sources describe Lughnasadh as a time of bilberries — people climbed hilltops to gather the first bilberries of the season and leave offerings. The practice of climbing a mountain or high hill on the first Sunday of August — Garland Sunday or Fraughan Sunday — survived into the modern period in Ireland, most famously as the pilgrimage to Croagh Patrick in County Mayo, where tens of thousands climb the holy mountain on the last Sunday of July each year.