In the Roman calendar the ides of March falls on March 15. The ides was an auspicious day in the Roman calendar, falling on the 15th of March, May, July and October and on the 13th of the other months.

The date is famous because Julius Caesar was assassinated on the ides of March, 44 BC. According to the Roman writer Plutarch, Caesar had been warned of the danger but had disregarded the warning:

What is still more extraordinary, many report that a certain soothsayer forewarned him of a great danger which threatened him on the ides of March, and that when the day was come, as he was going to the senate-house, he called to the soothsayer, and said, laughing, "The ides of March are come"; to which he answered, softly, "Yes; but they are not gone."

The soothsayer's warning to Julius Caesar, "Beware the Ides of March," has forever imbued that date with a sense of foreboding. But in Roman times the expression "Ides of March" did not necessarily evoke a dark mood—it was simply the standard way of saying "March 15." Surely such a fanciful expression must signify something more than merely another day of the year? Not so. Even in Shakespeare's time, sixteen centuries later, audiences attending his play Julius Caesar wouldn't have blinked twice upon hearing the date called the Ides.

The term Ides comes from the earliest Roman calendar, which is said to have been devised by Romulus, the mythical founder of Rome. Whether it was Romulus or not, the inventor of this calendar had a penchant for complexity. The Roman calendar organized its months around three days, each of which served as a reference point for counting the other days:

   * Kalends (1st day of the month)
   * Nones (the 7th day in March, May, July, and October; the 5th in the other months)
   * Ides (the 15th day in March, May, July, and October; the 13th in the other months) 

The remaining, unnamed days of the month were identified by counting backwards from the Kalends, Nones, or the Ides. For example, March 3 would be V Nones—5 days before the Nones (the Roman method of counting days was inclusive; in other words, the Nones would be counted as one of the 5 days).