Lughnasadh, Lughnasa or Lúnasa (/ˈluːnəsə/ LOO-nə-sə, Irish: [ˈl̪ˠuːnˠəsˠə]) is a Gaelic festival marking the beginning of the harvest season. Historically, it was widely observed throughout Ireland, Scotland, and the Isle of Man. Traditionally, it is held on 1 August, or about halfway between the summer solstice and autumn equinox. In recent centuries, some celebrations have shifted to Sundays near this date. Lughnasadh is one of the four Gaelic seasonal festivals, along with Samhain, Imbolc, and Beltane. It corresponds to the Welsh Gŵyl Awst and the English Lammas.

Lughnasadh is mentioned in early Irish literature and has pagan origins. The festival is named after the god Lugh. In the Middle Ages, it involved great gatherings that included ceremonies, athletic contests (most notably the Tailteann Games), horse racing, feasting, matchmaking, and trading. According to folklorist Máire MacNeill, evidence suggests that the religious rites included an offering of the First Fruits, a feast of the new food, the sacrifice of a bull, and a ritual dance-play. In recent centuries, Lughnasadh gatherings have typically been held atop hills and mountains, including many of the same activities.

The festival persisted widely until the 20th century, with the event called Garland Sunday, Bilberry Sunday, Mountain Sunday, and Crom Dubh Sunday. The tradition of climbing hills and mountains at Lughnasadh has survived in some areas and is recast as a Christian pilgrimage. The best known is the Reek Sunday pilgrimage to the top of Croagh Patrick on the last Sunday in July. Several fairs are also believed to be survivors of Lughnasadh, such as the Puck Fair. Since the late 20th century, Celtic neopagans have observed Lughnasadh, or something based on it, as a religious holiday. In some places, festival elements have been revived as a cultural event.

In Old Irish the name was Lugnasad (Modern Irish: [ˈl̪ˠʊɣnˠəsˠəd̪ˠ]). This is a combination of Lug (the god Lugh) and násad (an assembly), which is unstressed when used as a suffix. Another theory is that it originated from the word nás (death), rather than násad. Later spellings include Luᵹ̇nasaḋ, Lughnasadh and Lughnasa.

In Modern Irish, the spelling is Lúnasa [ˈl̪ˠuːnˠəsˠə], which is also the name for August. The genitive case is also Lúnasa as in Mí Lúnasa (Month of August) and Lá Lúnasa (Day of Lúnasa). In Modern Scottish Gaelic (Gàidhlig), the festival and the month are both called Lùnastal [ˈl̪ˠuːnəs̪t̪əl̪ˠ]. In Manx (Gaelg), the festival and the month are both called Luanistyn [ˈluanɪstθən]. The day itself may be called either Laa Luanistyn or Laa Luanys.

In Welsh (Cymraeg), the day is known as Calan Awst, originally a Latin term, meaning the Calends of August. In Breton (brezhoneg), the day was known as Gouel Eost, the Feast of August.

In Irish mythology, Lughnasadh is said to have been founded by the god Lugh as a funeral feast and athletic competition—funeral games—to commemorate the death of an earth goddess. Irish myths about Lughnasadh and Lughnasadh sites tend to feature a woman who is carried off or held against her will, and who dies of grief, shame, exhaustion, or unspecified natural causes. Parallels with the Greek tale of Persephone have been noted. A story about the Lughnasadh site of Tailtin says the festival was founded by Lugh as funeral games in memory of his foster-mother Tailtiu. She was said to have died of exhaustion after clearing the plains of Ireland for agriculture. Tailtiu may have been an earth goddess who represented the dying vegetation that fed mankind. Another tale, about the assembly site of Naas, says that Lugh founded the festival in memory of his two wives, the sisters Nás and Bói. One theory is that it was a mourning (or wake) for the end of summer.

Folklorist Máire MacNeill extensively studied the later folklore and traditions of Lughnasadh. She concludes that the main theme is a struggle for the harvest between two gods. One god, usually called Crom Dubh in later folklore, guards the grain as his treasure. The other god, Lugh, must seize it for mankind. Sometimes, this was portrayed as a struggle over a woman called Eithne, who represents the grain. Lugh also fights and defeats a figure representing blight. MacNeill says that these themes can be seen in earlier Irish mythology, particularly in the tale of Lugh defeating Balor, which seems to represent the overcoming of blight, drought and the scorching summer sun. In surviving folklore, Lugh is usually replaced by Saint Patrick, while Crom Dubh is a pagan chief who owns a granary or a bull and who opposes Patrick, but is overcome and converted. Crom Dubh is likely the same figure as Crom Cruach and shares some traits with the Dagda and Donn. He may be based on an underworld god like Hades and Pluto, who kidnaps the grain goddess Persephone but is forced to let her return to the world above before harvest time.