The Irish calendar is a pre-Christian Celtic system of timekeeping used during Ireland's Gaelic era and still in popular use today to define the beginning and length of the day, the week, the month, the seasons, quarter days, and festivals. The Irish calendar 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 Irish calendar fall around the solstices and equinoxes. As a result, for example, midsummer falls on the summer solstice. The beginnings of the seasons are roughly the halfway points between solstice and equinox:
This is a continuation of the Celtic and Gaelic system, which is pagan in origin. This is particularly evident in the Irish (Gaeilge) names for May (Bealtaine), August (Lúnasa) and November (Samhain), which were the names of Gaelic pagan festivals. In addition, the names for September (Meán Fómhair) and October (Deireadh Fómhair) translate directly as "middle of autumn" and "end of autumn". Christianity has also left its mark on the Irish months: the name for December (Nollaig) means Christmastide.
Historical texts suggest that, during Ireland's Gaelic era, the day began and ended at sunset.[1] Through contact with the Romans, the seven-day week was borrowed by continental Celts, and then spread to the peoples of Britain and Ireland.[1] In Irish, four days of the week (Monday, Tuesday, Saturday, Sunday) have names derived from Latin. The other three relate to the fasting done by Catholic clergy.[2]