Con il termine “idi”, i romani si riferivano al 13 o al 15 del mese, secondo il particolare calendario giuliano introdotto nel 45 a.C., il quale prevedeva tre date fisse per ciascun mese, in base a cui venivano stabiliti i restanti giorni. In particolare, le idi di marzo, che nell’antica Roma coincidevano con il primo giorno di primavera, ovvero il 15 marzo, sono una data divenuta storicamente celebre poiché coincide con l’uccisione di Giulio Cesare nel 44 a.C., a seguito di una congiura portata avanti da un gruppo di circa 60 senatori, tra cui Bruto e Cassio, contrari ad ogni forma di potere personale del dittatore, che nel corso degli anni aveva ottenuto numerose cariche che avrebbero minacciato il potere del senato. La morte di Giulio Cesare, pugnalato ventitré volte in occasione di una seduta senatoria, divenne oggetto di un dibattito culturale che la vedeva da un lato come giusta difesa delle istituzioni della repubblica, minacciate da un tiranno, dall’altra come un gravissimo tradimento.

In modern times, the Ides of March is best known as the date on which Julius Caesar was assassinated; stabbed to death at a meeting of the senate in 44 B.C. As many as 60 conspirators, including Brutus and Cassius, were involved. According to Plutarch, the Greek biographer and essayist, a seer had warned that harm would come to Caesar no later than the Ides of March. Upon his journey to the theatre where Caesar would ultimately meet his death, he passed the seer and mocked “The Ides of March are come,” implying that the prophecy had not been fulfilled. The seer replied, “Aye, Caesar; but not gone.”

The senate meeting was famously dramatized in William Shakespeare’s play “Julius Caesar;” the origin of the saying “Beware the Ides of March.” In Roman history, the Ides of March was a turning point; one of the events marking the transition from the Roman Republic to the Roman Empire. The Ides fall on the 15th of the month. The Romans did not number days of the month sequentially. Rather, they counted back from three fixed points of the month: the Nones (5th or 7th depending on the length of the month), the Ides (13th or 15th) and the Kalends (1st of the following month). For most months, the Ides occurred on the 13th, but in March, May, July, and October the Ides fell on the 15th. The Ides were supposed to be determined by the full moon; on the earliest calendar, the Ides of March would have been the first full moon of the new year (in the oldest Roman calendar, March was the first month of the year). The Ides of March marked several religious observances as well. The Ides of each month was sacred to Jupiter, the Roman’s supreme deity. Jupiter’s high priest led the “Ides sheep” in procession to the citadel where it was sacrificed. The Ides of March was also the Feast of Anna Perenna, a goddess of the year. The festival was celebrated among the common people with picnics, drinking and revelry. Today, Rome hosts cultural events and reenactments of Caesar’s death on the Ides of March.