In the days of ancient Rome, the Ides were certain days on the Roman calendar, usually the 13th day of the month, but the 15th of the month in March, May, July and October. The Ides marked the middle of the month, and used to be marked by the Full Moon, as the Roman calendar was originally lunar based. The oldest Roman calendar started with Martius, now known to us as March – this would make the Ides of March the first Full Moon of the New Year, as reckoned by our ancient Roman ancestors.
The negative connotation of the Ides of March comes from the death of Julius Caesar on March 15th in the year 44 BC.
Julius Caesar had been warned by a soothsayer (thought to be a haruspex*, or someone who inspected the entrails [usually, livers] of sacrificed animals) that danger would befall him before the Ides of March have passed. When that day came – the 15th of March in 44 BC – Caesar passed the soothsayer and said tauntingly “The Ides of March have come”, implying that the prophecy was false, but to which the soothsayer replied “Aye, Caesar, the Ides are come but not gone.” Shortly thereafter Julius Caesar was assassinated by dozens of those who had conspired against him. Caesar’s death by betrayal was a period of change – the Roman Republic became the Roman Empire.
This is recounted in Shakespeare’s play Julius Caesar, which made famous the phrase “Beware the Ides of March”.
*Haruspicy is a form of divination where the livers of sacrificed animals are inspected for certain signs. This ancient practice is mentioned in the Bible, in the book of Ezekiel, chapter 21, verse 21:
“For the king of Babylon stands at the parting of the way, at the head of the two ways, to use divination; he shakes the arrows, he consults the household idols, he looks at the liver.”