The Ides of March was a date on the ancient Roman calendar. In a play by William Shakespeare, the dictator Julius Caesar was told by prophets to beware this date for he will meet his doom on this date.
During her attempt to kill Captain James T. Kirk, whom she had compared to Caesar, Lenore Karidian cited a line from this play, "Caesar, beware the Ides of March", before firing her phaser. Her attempt failed when her father, Anton Karidian, placed himself before the captain and took the fatal shot. (TOS: "The Conscience of the King")