The phrase 'Beware the ides of March' is a warning from William Shakespeare's play Julius Caesar. The term 'ides' refers to the 15th day of the month in the ancient Roman calendar, and the warning was given to Julius Caesar by a soothsayer who warned him to 'beware the ides of March'. In the play, the warning is a sign of impending doom for Caesar, as he is assassinated on the ides of March.