The Ides is a day that comes every month, not just in March, according to the ancient Roman calendar. The Romans tracked time much differently than we do, with months divided into groupings of days counted before certain named days—the Kalends at the beginning of the month, the Ides at the middle, and the Nones between them. In a 31-day month like March, the 15th serves as the Ides.

In 44 BCE Julius Caesar was in the midst of a series of political and social reforms when he was assassinated by a group of nobles on the Ides of March. Led by senators Marcus Junius Brutus and Gaius Cassius Longinus, a group of approximately 60 conspirators fatally stabbed Caesar in the Roman Senate in a plot to preserve the Roman Republic and halt Caesar’s increasingly monarchical regime.

The term was immortalized in the tragedy Julius Caesar by William Shakespeare. In the play, a soothsayer (or fortune teller) warns Caesar to “beware the Ides of March,” but the dictator dismissed the advice, saying, “He is a dreamer; let us leave him: pass.” Later in the play, as Caesar approaches the Senate House, he sees the soothsayer in a crowd and boasts, “The Ides of March are come,” to which the soothsayer replies, “Ay, Caesar; but not gone.”