Lughnasadh (pronounced LOO-nə-sə; Irish: Lúnasa; Scottish Gaelic: Lùnastal; Manx: Luanistyn) is a traditional Gaelic holiday celebrated on 1 August in the northern hemisphere and 1 February in the southern. It originated as a harvest festival, corresponding to the Welsh Calan Awst and the English Lammas.

In Old Irish (or Old Gaelic), the name was usually spelt Lugnasad (pronounced [luɣnəsəð]). Later spellings include Luġnasaḋ, Lughnasadh and Lughnasa.

In Modern Irish (Gaeilge), the spelling is Lúnasa, which is also the name for the month of August. The genetive case is also Lúnasa as in Mí Lúnasa (Month of August) and Lá Lúnasa (Day of Lúnasa). The word násadh means a feast, fair, assembly, or celebration, but is unstressed when used as a suffix on Lughnasadh.

In Modern Scottish Gaelic (Gàidhlig), the festival and the month are both called Lùnastal.

In Manx (Gaelg), the festival and the month are both called Luanistyn. 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, the Kalends of August in English.