Lughnasadh (or Lughnasa; modern Irish Lúnasa) is a Gaelic holiday celebrated on 1 August, during the time of the harvesting. Lugnasadh was one of the four main festivals of Celtic religion: Imbolc, Beltaine, Lughnasadh and Samhain. Lughnasadh means "Lugh's assembly", representing the last festival of the calendar, dedicated to Lugh, the Sun God of Celtic mythology. The name Lammas is also used, taken from an Anglo-Saxon and Christianized holiday occurring at the same time, that may or may not have a common origin. As the name (from loaf-mass, "loaves festival") implies, it is a feast of thanksgiving for bread, symbolizing the first fruits of the harvest. Lughnasadh festivals lasted from 15 July until 15 August. Aside from three days of religious rituals, the celebrations were a time for contests of strength and skill.

Some Irish people continue to celebrate the holiday with fires and dancing. Lughnasadh is also the modern Gaelic term for the month of August.

In neopaganism, Lughnasadh is one of the eight sabbats or solar festivals in the Wheel of the Year. It is the first of the three autumn harvest festivals, the other two being Mabon and Samhain. It commemorates the sacrifice and death of the Corn God; in its cycle of death, nurturing the people, and rebirth, the corn is thought of as an aspect of the Sun God. Some Neopagans mark the holiday by baking a figure of the God in bread, and then symbolically sacrificing and eating it.

Lughnasadh is often defined as a cross-quarter day midway between the summer solstice and the autumn equinox, which is half way through Leo (in the northern hemisphere) or Aquarius (in the southern hemisphere). Lughnasadh in the northern hemisphere coincides with Imbolc in the southern hemisphere. As a sabbat it is preceded by Midsummer and followed by Mabon.

There is a play by Brian Friel entitled Dancing at Lughnasa which has also been made into a 1998 movie.

See also: autumn equinox, Samhain, winter solstice, Imbolc, spring equinox and summer solstice.