In my opinion, the primary reason we know the ides of March is that Shakespeare incorporated the phrase “beware the ides of March” into Julius Caesar, a soothsayer’s warning to Caesar. His impending assassination by Brutus and Cassius would take place on March 15. But the origin of the phrase, as far as I know, is from Plutarch’s Life of Julius Caesar, from which Shakespeare borrowed and adapted for dramatic effect.
Something I learned only recently, though I do not recall exactly when or why, is that “ides” does not refer to the fifteenth of the month but, rather, to a day that was calculated to coincide with the full moon. According to an explanation of the Roman calendar on timeanddate.com:
“Ides occurred on the 15th day of March, May, July, and October, and the 13th day of the other months. They are thought to have been the days of the full moon…each day was referred to by how many days it fell before the Calends, Nones or Ides. For example, March 11 would be known as “Five Ides” to the Romans because it is four days before the Ides of March (March 15).”
I’d like to think I need not beware of the ides of March in this or any other year, at least not in the same sense that Julius Caesar should have been. To my knowledge, no one is plotting my assassination on that, or any other day. If you have knowledge to the contrary, please contact me.