The Ides of March refers to the 15th day of the month of March. In ancient Rome, all months had days with specific designations, and the Ides of any month was one of these. It indicated roughly the middle of the month, and probably also referred to the day of the full moon. The Ides of March in particular was a festival day during which Romans would participate in or observe a military parade dedicated to Mars, the god of war.

Other significant days in the Roman calendar were the Kalends, or the first of the month, and the Nones, which was either the seventh or the fifth day, depending upon which month it was. Each day of the calendar was named according to its relationship to one of these specially designated days, rather than by its actual number in the month. For example, the second day of March would be referred to as VI Nones, indicating it was six days before the Nones of March. In spite of the confusing nature of this system, its use continued well into the Middle Ages in some areas.

So if all ten months of the Roman calendar had an Ides, why is the Ides of March the only one that retains significance in modern times? This is largely due to the occurrence of a significant historical event on that date in 44 BC. On the March 15, Julius Caesar was murdered on the senate floor, betrayed by Marcus Junius Brutus, Gaius Cassius Longinus and sixty other conspirators who wished to see the powerful leader removed from his office. They stabbed him twenty-three times—probably more than was necessary, but certainly enough to ensure the assassination attempt was successful. 

Plutarch, a Roman historian who chronicled the lives of many Roman emperors, wrote a biography of Julius Caesar as part of his Parallel Lives series. In these books, he wrote a biography of a Greek and a Roman leader, comparing the two. Paired with Julius Caesar was Alexander the Great. His account of the life of Julius Caesar is the best-known and most complete biography of the leader, although it was written over a century after his death. Much of Plutarch’s version appears to have been derived from The Twelve Caesars, a biographical work written by Gaius Suetonius Tranquillus in AD 121.

In Plutarch’s biography, he mentions that a seer warned Caesar that he would be harmed on or before the Ides of March. Whether or not this is true, the idea was picked up by William Shakespeare later, when he wrote his play, Julius Caesar. The line “Beware the Ides of March” entered into common usage via Shakespeare’s dramatic interpretation.

The combination of these three events—Caesar’s actual murder on the Ides of March, Plutarch’s dramatization of the event, with added prognostication, and Shakespeare’s use of the idea—the Ides of March have come down in history as a date to be eyed with caution, even for those not likely to be stabbed to death on the floor of the ancient Roman senate.