The term ‘Ides’ had a perfectly practical – and in no way sinister – meaning to the Romans. Their calendar, in connection with the phases of the Moon, used three terms to mark the passage of a month. A ‘Kalend’ was the first day of the month; a ‘None’ signified the first quarter of the Moon (the fifth or seventh day) and an ‘Ide’ was the full Moon (the 13th or 15th day). Therefore the Ides of March simply meant: 15 March.
In 44 BC, the Ides of March took on a new meaning. Julius Caesar, who had seized power from the Roman Republic and made himself a dictator, was murdered by a group of 60 dagger-wielding Senators led by his friends, Brutus and Cassius. Caesar knew many wished him dead and a soothsayer allegedly warned him that harm would come to him before the Ides of March. On the 15th, Caesar passed the soothsayer joking, “The Ides of March have come,” but was met with the ominous reply, “Aye, Caesar, but not gone.”
This moment has been immortalised by William Shakespeare in his play, Julius Caesar. It is from Shakespeare that we have the famous warning given by the soothsayer, “Beware the Ides of March”.
Caesar’s murder failed to bring back the Republic, but triggered the start of the Roman Empire under his adopted heir, Octavian (known as Augustus). Before the assassination, the thing the Ides of March was best known for was a festival. The popular time for feasting and drinking marked the festival of Roman deity Anna Perenna. It was also traditionally the time to settle debts – Julius Caesar certainly paid his. Via What are the Ides of March?