The Feast of Lughnasadh falls on the cross-quarter day between the summer solstice and the autumn equinox. The festival was, according to legend, instituted by the god Lugh on the occasion of his foster-mother Tailtiu's death as a funeral feast. It is clear, however, that the feast originated as a commemoration of Lugh's slaying of the giant Balor, and the prizing of agricultural secrets from the dark god Bres. This victory over the Fomorians marked the beginning of the rule of the Tuatha Dé Danann.

In the calendrical pageant of the gods, Lughnasadh commemorates this victory over darkness and celebrates the fruit of that victory, the first fruits of the harvest. It can be looked at as the victory of Lugh, the gentler solar aspect, over Balor, his harsher aspect. The feast of Lugh is held at the hottest time of the year, when the battle against the harshness of the sun was often very real.

Tailtiu seems to have been an ancient agricultural goddess, about whom little is known except for the harvest festival held in her honor. In Lugh's story, she is the Fir Bolg princess who fosters him. During the period of starvation that marked the captivity of the Tuatha Dé Danann by the Fomorians, she performs heroically, clearing a massive plain for planting so that the people will not starve. She dies as a result of her exertions, and the grieving god institutes a funeral feast in her honor. The month of August in Ireland has long been associated with sorrow and mourning — this may reflect a tradition of mourning for the “sacrifice” of the grain.

Lugh was called “many skilled,” and his feast was a pageant of skill as well, from feats of athleticism and competitive games to displays of handicrafts and metalwork. Lughnasadh represented the last period of rest before the major work of the agricultural year began. It was a time for trade between tribes, for sport and friendly competition, and for reciting songs of praise for the heroic deeds of the heroes and the fallen warriors of the tribes. The crops were ripened, and there were trades and goods to be purchased with the new bounty. Lughnasadh was also a popular time for new couples to enact temporary partnerships called “Tailtian marriages.” These trial marriages were solemnized at the festival and typically lasted until the next year.

These traditions in particular remained strong through Christian times. Predictably, the church discovered yet another festival to commemorate, which just happened to fall at this time — the Lammas festival. Lammas also celebrated the harvest, and it was celebrated in much the same way, with games and contests and open-air markets. The name Lammas is believed to be derived from “loaf-mass,” but this explanation is not universally accepted.