The term Ides derives from the Latin word iduare (“to divide”), with the full moon serving as the division point in the middle of each month. In the ancient Roman calendar, months were divided according to the lunar cycle into three groups of days. The Ides corresponded with the rise of the full moon in the middle of the month. Depending on the length of the month, the Ides fell on the 13th or 15th day. And since the Roman new year began in March, the Ides of March marked the first full moon of the year.

In 44 BCE Roman ruler Julius Caesar 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. His death triggered a civil war that led to the rise to power of his great-nephew and adopted son, Octavian, who became the first Roman emperor, Augustus Caesar, in 27 BCE.

In Act 1, Scene 2 of Shakespeare’s Julius Caesar, a soothsayer (a type of fortune teller) warns Caesar to take heed of the Ides of March. Caesar dismisses him: “He is a dreamer; let us leave him: pass.” The scene indicates Caesar’s arrogance. Act III opens with Caesar approaching the Senate House on the Ides of March. He notices the soothsayer in a crowd and boasts “The Ides of March are come,” to which the soothsayer replies, “Ay, Caesar; but not gone.”