Lughnasadh is a festival representative of the first harvest of the year, typically held on August 1st about midway between the summer solstice and autumn equinox. It is one of four Gaelic festivals historically celebrated throughout Ireland, Scotland and the Isle of Man with variations in other Celtic lands such as Wales. The remaining three festivals are Samhain, Imbolic and Beltane.
Samhain occurs on October 31st, about midway between the autumn equinox and the winter solstice. Imbolic is traditionally held around February 1st, about midway through the winter solstice and the spring equinox. Beltane, also known as May Day, occurs on May 1st roughly between the spring equinox and the summer solstice. It's safe to assume that these festivals had influences on other religions and cultures alike such as modern day Halloween and Candlemas (perhaps not quite as culturally popular as of Groundhog Day in the US and Canada).
Back to Lughnasadh - rooted deep in Celtic mythology, the legend consists of a Celtic god named Lugh, who the festival honors in part. As important, this day was additionally in remembrance of his stepmother, Tailtiu, who had passed away after clearing lands throughout Ireland for planting crops. Lughnasadh, like many Celtic festivals, included a feast and this one in particular was a celebration of the first harvest. Aside from feasting, there were a number of other components to the festival. Athletic contests were prominent as well as singing, dancing and matchmaking.
Through the centuries, there have been variations with the events that occur during the festival and equally depending on which area within the Celtic nations it took place. It wasn't only the Celts which adopted this festival; modern day pagans and polytheists celebrate similar versions of Lughnasadh as well.
Lughnasadh is translated into Irish, Scots Gaelic and Manx as Lúnasa, Lùnastal and Luanistyn, respectively.