ides: The 15th of March, May, July, or October or the 13th day of the other months in the ancient Roman calendar. (From the Latin idus, a possible derivative of an Etruscan word meaning “division of a month.”)

Apparently, the Romans only considered three days a month important: Kalends (the first day of the month), Nonas (the 7th of March, May, July, or October or the 5th of the other months), and Ides. You then counted backward from those dates to keep track of the other days of the month.

The dates were lunar and monetary in origin. Kalends, from which the word “calendar” is derived, itself came from the word Kalendrium, meaning “account book,” and was the day on which debts were due. Nonas was traditionally the day on which the moon was half-full. And Ides was traditionally the day of the full moon.

The difference in dates (5th vs. 7th and 13th vs. 15th) seems to correspond to how long the months were in the Roman calendar. March, May, July, and October were 31-day months, while the rest were 28 or 29 days long.