Other Crap

Beware the Ides of March. Wait, what the fuck is an Ide anyway?

OED weighs in: "In the ancient Roman calendar, the eighth day after the nones, i.e. the 15th of March, May, July, October, and the 13th of the other months. The days after the nones were reckoned forward to the ides; hence such expressions as ‘the sixth of the ides’ (or ‘the sixth ides’, or ‘the sixth ide’) ‘of June’, loosely rendering L. ante diem sextum Idus Junias = June 8.

The original Roman Calendar was based on the moon, and all months were the same length. The full moon was called Kalends and began the month, and the new moon was called Ides. There was another marker called Nones which was actually nine days before the new moon. They counted the time forward until the next marker. Thus, the day before the Ides was called "pre-Ides", and the day before that was called "third day of Ides" (not the second. Ides itself was considered part of the cycle), and so backwards to the "eighth day of Ides." The day before that was the "Nones."

By the time of Caesar, the Romans had adopted a standardized solar calendar, with months of varying lengths, but the old terms were kept as part of tradition, albeit with a slightly different meaning. The three "marker" days retained special significance. The Kalends kicked off every month, and the Ides marked the approximate halfway point. (To be technically correct, it occurred on the 13th in 30 day months, and the 15th in 31 day months.) The Nones remained exactly nine days before the Ides

It was a clumsy system, May 21 would be called "ante diem XII Kal. Jun," literally "the twelfth day of Kalends of June, counting backwards" but rendered into modern English "eleven days before June 1."

Here's OED's history of usage and spelling:

c1330 R. Brunne Chron. (1810) 341 Idus þat is of May left I to write þis ryme, B letter & Friday bi ix þat Šere Šede prime.

1483 Caxton Gold. Leg. 215b/2 It was the iii ydees of Juyll.

1502 Arnolde Chron. 65b/2 Somer+beginnithe the vij. Ide of may and lastith vnto the vij. Ide of august.

1576 Fleming Panopl. Epist. 40 Dated the .7. of the Ides of June.

1601 Shakes. Jul. C. i. ii. 17 Cæsar+Beware the Ides of March.

1641 Howell Vote in New Vol. Lett. (1650) Iij, The soft gliding Nones and every Ide.

1776 A. Adams in J. Adams' Fam. Lett. (1876) 160 The 19th of April, ever memorable for America as the Ides of March to Rome and to Cæsar.

1834 Lytton Pompeii i. iii, ‘It stands fixed for the ninth ide of August’, answered Pansa.

1847 Emerson Woodnotes i. 45 Foreteller of the vernal ides, Wise harbinger of spheres and tides.