Believe it or not, the Ides of March is simply an old-fashioned — ancient Roman, to be exact — way of saying March 15. The word "Ides" comes from the earliest Roman calendar, which organized its months around three days — Kalends, Nones and Ides — each of which was used as a reference point for counting the other days of the month.
Kalends (from which the English word calendar is derived) was the first day of each month. Nones was the seventh day in March, May, July and October and the fifth day in every other month. Ides was the 15th day in March, May, July and October and the 13th day in every other month.
The rest of the days each month were identified by counting backward from the Kalends, Nones or Ides. For example, March 11 would be V Ides, or five days before the Ides of March. (V is the Roman numeral for five.) Note that the Roman way of counting was inclusive, so the Ides was counted as one of the five days.
Historians suspect Ides might refer to the day of the full moon, since it is believed that the ancient Roman calendar was based on the phases of the moon.
So how did a minor ancient Roman day of festivities come to have such modern cultural significance? Today, the Ides of March commonly refers to March 15, 44 B.C., the day Julius Caesar was assassinated in the Roman Senate.
Of course, the seer's prediction came true not long after, when Brutus, Cassius and more than 60 other co-conspirators stabbed Caesar to death. Caesar's meeting with the seer was popularized in William Shakespeare's play Julius Caesar, in which Caesar is warned to "beware the Ides of March."