Lughnasadh: LOO-ne-sah (thanks
ryano for the pronunciation)
1. August in the Irish and Scottish calendar, sometimes spelled Lunasa after the language reforms of the early twentieth century.
2. August 1. Irish feast day of Lugh, supposedly established by Lugh after he defeated the Fomorians. According to the "Second Battle of Magh Turedh" in the Book of Invasions, Lugh established this holiday in honor of his foster mother Tailtiu, an earth goddess. It was celebrated with athletic competition (much like the Olympics) and was the first harvest. This is likely tied to Lugh wrestling the secrets of agriculture from Bres, as shown in 2MT.
As Tailtiu was an earth goddess, this is the first harvest festival of the Celts, the second being Samhain. In later times, this was Christianized as Lammas--"Loaf Mass," celebrating the Eucharist--in the Saxon areas. As there is no record of a Germanic festival occuring at August 1, both Maire MacNeill and Roland Hutton believe that the origin of Lammas would be a Celtic festival held around August 1--in Irish this is Lughnassadh.
There were other names and traditions for the holiday, celebrated on or around August 1; these included Crom Dubh Sunday (when Patrick battled the demon Crom Dubh), Garland Sunday, and in Old Irish, Bron Trogain--"the labor pains of the earth", refering to harvest.
SOURCES
Hutton, Ronald. The Stations of the Sun: A History of the Ritual Year in Britain.
MacNeill, Maire. The Festival of Lughnasa. OUP, 1962.