Today is March 15 – the Ides of March.

And, chances are, you may hear someone warning you to beware today. Ever wondered why?

The Ides of March is the day on the Roman calendar that corresponds to March 15. It marks the day in 44BC that Roman leader Julius Caesar was assassinated by a group of Senators he considered allies. Caesar had been warned by a seer that he would come to harm no later than the Ides of March, or March 15. On his way to the Senate, Caesar supposedly joked “The ides of March have come,” to which the seer replied “Aye, Caesar, but not gone.”

Minutes later, Caesar was dead.

March 15 also marked the time when Roman citizens paid outstanding debts, making it another red-letter day to many people.

The combination of Caesar’s assassination and a version of Roman tax day gave March 15 its own historical black eye. It also marked the time of great transformation, as Caesar’s death was the central event in marking the transition from Roman Republic to the Roman Empire.

Fast forward to the 16th Century. Playwright William Shakespeare uses the legend of the Ides as a central point in his play about Julius Caesar. In his work, a soothsayer cautions Caesar to “beware the Ides of March” shortly before the emperor is murdered in the Senate.

Shakespeare’s turn of phrase lives on today, giving the Ides its bad reputation.

Not everyone believes March 15 is an unlucky day, or even its origin. You can see more Ides of March myths here.