Lughnasa (Lúnasa)[see also festivals], the day marking the beginning of autumn, identified with 1 August in the Julian calendar. A survival of the ancient harvest festival, it is named after the god Lug, as noted in the 9th-cent. Sanas Chormaic [see Cormac mac Cuilennáin and glossaries].

unreformed ModIr., Lugnasad OIr., Lughnasadh, Lúnasa reformed ModIr., Lùnasdain, Lùnasdal, Lunasduinn ScG, Laa Luanistyn, Laa Luanys Manx;

Also Lammas Day, Garland Sunday, Domhnach Chrom Dubh, Crom Dubh Sunday, Bilberry Sunday, Fraughan Sunday [Irish Lug; násad, assembly, festive or commemorative gathering]. Irish, Scottish Gaelic, and Manx names for the seasonal feast of pre-Christian origin fixed at 1 August in Ireland and on the Isle of Man, the first Sunday in August or the last Sunday of July in the Gregorian calendar; in Scotland the festival is renamed in honour of St Michael, Michaelmas, 29 September. One of the four great calendar feasts of Celtic tradition along with Samain (1 November), Imbolc (1 February), and Beltaine (1 May). Long a harvest festival celebrating the ripening of grain and, after they became plentiful, the maturing of potatoes, Lughnasa commemorates Lug Lámfhota, one of the most prominent heroes of early Irish literature. Lug seems most certainly derived from Lugos/Lugus or Gaulish Mercury, the god described by Julius Caesar (1st cent. BC) as the most prominent in the Gaulish pantheon. At Lug(o)dunum (Lyon), a city named for Lugos/ Lugus, a celebration was held each 1 August in honour of the Emperor Augustus. According to early Irish tradition, however, Lug himself established the festival to honour his foster-mother Tailtiu in Brega, modern Co. Meath; Lug also led the horse-racing and martial arts contests. Soon Lughnasa celebrations were held in other parts of Ireland, at Emain Macha for Ulster and at Tara for the whole of the island.

The Christian Church did not oppose the continuation of the festival marking the beginning of the harvest and the weaning of calves and lambs, but the different names applied to it obscured its pagan origin. Eventually it broke away from its fixed time of 1 August and might include many days, especially Sundays, from 15 July to 15 August. Comparable but much smaller fairs, not associated with Lug or Lughnasa, were held for Calan Awst [first of August] in Wales and at Morvah in Cornwall. Lughnasa and its counterparts provided a time for horse-racing, horse-swimming, and games of hurling. Celebrants might enjoy climbing to the tops of nearby hills, both to pray and to gather bilberries. Others would assemble at lakes and holy wells. Lughnasa fairs might also include the buying and selling of goods, especially at Killorglin in Co. Kerry, Ballycastle in Co. Antrim, Ennistymon in Co. Cavan, and elsewhere. Máire MacNeill's landmark study, The Festival of Lughnasa (Oxford, 1962), details the persistence and extent of celebrations. See also Pádraig Ó Riain, ‘Traces of Lug in Early Irish Hagiographical Tradition’, Zeitschrift für celtische Philologie, 36 (1977), 138–56; T. J. Westropp, ‘Marriages of the Gods at the Sanctuary of Tailltiu’, Folk-Lore, 31 (1920), 109–41. Brian Friel's drama Dancing at Lughnasa (1990) draws thematically on festival traditions.