ides

Etymology 1

From Middle English ides, idus, from Anglo-Norman and Old French ides, from Latin īdūs, a fourth-declension plurale tantum, from the Latin practice of treating most recurring calendrical days as plurals. The Latin term is cognate with Oscan eiduis, both perhaps deriving from an unknown Etruscan term. Middle English and Old French also used the singular form ide.

noun

1. (historical, often capitalized) The notional full-moon day of a Roman month, occurring on the 15th day of the four original 31-day months (March, May, Quintilis or July, and October) and on the 13th day of all other months.
   Þa monðas þe habbað iiii nonas æfter kalendas... habbað to idus xiii dagas and to ii kalendas eahtatyne. Those months that have 4 nones after the kalends... have 13 days to the ides and eighteen to the second kalends. 10th century, Byrhtferð of Ramsey, Enchiridion (Ashmolean MS 328), Book I, Chapter ii, Section 22
   The Roman Month its several days divides By reckoning backwards, Calends, Nones, and Ides. 1679, J. Moxon, Mathematics made Easie, page 26
   For the modern reader of Latin the most irritating pecularity of this system of dating is that the days after the Ides of any month carry the name of the following month... Another trap for the unwary lies in the fact that the Roman calendars given in most reference books are Julian, not pre-Julian. When Caesar added ten days to the Roman year he put them near the ends of the seven 29-day months, one or two in each. As a result, instead of the day after the Ides of all months being a.d. XVII Kal., in these seven months it is either a.d. XVIII Kal. or a.d. XIX Kal., and all the following days change correspondingly. 1967, Agnes Kirsopp Michels, Calendar of the Roman Republic, page 22
   [March, May, Quintilis, and October] also have their Nones on the seventh, as Numa ordained, because Julius changed nothing about them. As for January, Sextilis, and December, they still have their Nones on the fifth, though they began to have thirty-one days after Caesar added two days to each, and it is nineteen days from their Ides to the following Kalends, because in adding the two days Caesar did not want to insert them before either the Nones or the Ides, lest an unprecedented postponement mar religious observance associated with the Nones or Ides themselves, which have a fixed date. 2011, Robert A. Kaster trans. Macrobius, Saturnalia, Book I, Chapter xiv, Section 8
   The third day before the ides of March is March 13th; the third ides of August is August 11th; and the third of the ides of November is November 11th.

Etymology 2

See ide.

noun

1. plural of ide