Lughnasa was celebrated the first day of August, during the time of the harvesting of the vegetables planted during the year. It has a patriotic character and was the festival that guaranteed peace and abundance of food.
Nowadays, after romanization and christianization of Europe, this pagan festival was associated with christian saints, and all the festivals that even today are celebrated during the first fifteen days of August have their root in this ancient festival.
There is a play by Brian Friel entitled Dancing at Lughnasa which has also been made into a 1998 movie.