This word was used in the Roman calendar. In months of 31 days (March, May, July, October), the Nones were the seventh day and the ides the fifteenth, while in the shorter months (all of the months except March, May, July, and October), the Nones fell on the fifth and the ides on the thirteenth day.
Plutarch (Greek biographer and philosopher; died ca. A.D. 120) in his Life of Julius Caesar wrote, "There was a certain soothsayer that had long before given Caesar warning to take heed of the Ides of March (which is the fifteenth of the month), for on that day he should be in great danger. The day having arrived, Caesar went to the Senate-house and spoke merrily to the soothsayer, saying: "The Ides of March is come."
"So it is," the soothsayer answered softly, "but yet it is not past."
In William Shakespeare's The Tragedy of Julius Caesar, he had essentially the same version in that Caesar was warned previously by a soothsayer to, "Beware the Ides of March!" On the day that Caesar was going to the Capitol with Antony, Brutus, Cassius, and others; he saw the same soothsayer who had warned him and since it is now the 15th of March, he remarks that the Ides have come, to which the soothsayer replied that they have not gone. Sure enough, Caesar was murdered on the Ides (15th) of March.