The Gaelic calendar, or alternatively the Irish calendar, is a system of timekeeping developed during Ireland's Gaelic era  In common with other Western cultures, contemporary Ireland utilizes the Gregorian calendar; however, the calendar developed in the Irish Gaelic era does not observe the astronomical seasons that begin in the Northern Hemisphere on the equinoxes and solstices, or the meteorological seasons that begin on March 1, June 1, September 1, and December 1. Rather, the middle of the seasons in the Gaelic calendar fall around the solstices and equinoxes. As a result, for example, Midsummer falls on the Summer Solstice. The pre-Christian Celtic year began on 1 November, although in common with the rest of the Western world, it now begins on 1 January.
- Winter ("Geimhreadh") - November, December, January (Samhain, Nollaig, Eanáir)
- Spring ("Earrach") - February, March, April (Feabhra, Márta, Aibreán)
- Summer ("Samhradh") - May, June, July (Bealtaine, Meitheamh, Iúil)
- Autumn ("Fómhar" Harvest) - August, September, October (Lúnasa, Meán Fómhair, Deireadh Fómhair)
As in other European languages, the names of the months in the Irish language bear evidence of religion and mythology which predates the arrival of Christianity. The words for May (Bealtaine), August (Lúnasa) and November (Samhain), were the names of Gaelic religious festivals. In addition, the names for September (Meán Fómhair) and October (Deireadh Fómhair) translate directly as "middle of harvest" and "end of harvest". Christianity has also left its mark on the Irish months: the name for December (Nollaig) derives from Latin natalicia (birthday), referring to the birth of Christ.
Historical texts[which?] suggest that, during Ireland's Gaelic era, the day began and ended at sunset. Through contact with the Romans, the seven-day week was borrowed by continental Celts, and then spread to the people of Ireland. In Irish, four days of the week have names derived from Latin, while the other three relate to the fasting done by Catholic clergy. Dé Luain; Dé Máirt; Dé Sathairn; Dé Domhnaigh.
- Dé Luain - from Latin dies Lunae
- Dé Máirt - from Latin dies Martis
- Dé Chéadaoin - referring to Catholic fasting: from céad (first) aoin (fast) i.e. the first fast of the week
- Déardaoin - the day between the fasts
- Dé hAoine - the day of the fast
- Dé Sathairn - from Latin dies Saturni
- Dé Domhnaigh - from Latin dies Dominicus (an alternative Latin name for Sunday, dies Solis being more common)
- See Nora Chadwick, The Celts (1970) p.181
- For instance, the word "February" in English derives from the Roman purification rite, Februa.
- Koch, John T. Celtic Culture: A Historical Encyclopedia. ABC-CLIO (2006). Page 330.
- Koch, John T. Celtic Culture: A Historical Encyclopedia. ABC-CLIO (2006). Page 331.