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 15^th day in March, May, July and October and the 13^th 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.

In ancient Rome, the Ides of March was a festive day dedicated to Mars, the god of war, after whom the month of March is named. Ancient Romans usually celebrated by holding a military parade.

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.

On his way to the Senate that day, Caesar met a “seer” (sometimes called a “soothsayer”) who predicted that harm would come to him no later than the Ides of March.

Caesar apparently did not take much stock in this prediction, noting that “the Ides of March have come.” The seer is said to have replied, “Ay, they have come, but they are not gone.”

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.”

Today, the phrase “the Ides of March” carries with it a sense of foreboding and is often used to signify a fateful day.