“Beware the Ides of March!” Everyone who has read or seen a production of Shakespeare’s Julius Caesar knows that warning, but not everyone knows what it means. According to one authority, the word Ides probably means something like “divider,” from the Etruscan verb iduare, meaning “to divide.”

The Ides divided Roman months approximately in half. In Julius Caesar’s time they occurred on the 15th day of 31-day months and the 13th day of the others. So Caesar’s Ides of March was March 15, the day on which he was assassinated in 44 B.C. If Caesar had been warned to beware of March 15, however, or even the 15th day of March, he would not have known what day to prepare for.

In 44 B.C., the Romans numbered their days according to an ancient system that derived from a primitive lunar calendar. They called the first day of each month the Kalends, which means “to proclaim.” It refers back to a time when priests proclaimed the beginning of a new month at the first visible crescent of the new moon.

In Caesar’s time, priests still proclaimed a new month on the Kalends, but months were no longer based on the moon. The priests merely announced how many days it would be until the next important day of the new month. This next important day was the Nones, which may once have been the day of the moon’s first quarter but by Caesar’s time was always scheduled for the ninth day before the Ides.

In the old lunar calendar, the Ides had been the day of the full moon, but by Caesar’s time it was simply the midpoint of the month. Because the lunar calendar was always looking forward to the next phase of the moon, its days counted downward to the anticipated day. Caesar’s days still counted downward too.

Therefore, the last few days before his assassination would have been numbered V Ides, IV Ides, III Ides, Day Before Ides, and Ides — not March 11, 12, 13, 14, and 15. When you think about it, this way of counting time has its merits.

We still count down to days that excite us — our birthdays, holidays, the last day of school. Maybe when we started counting upward, we lost a significant vestige of lunar influence that the Roman calendar still clung to — days that looked forward to the future rather than days that merely added up the past.