I've posted this half a dozen times before, but it's time of year for it again, so here you go:
The Ides of March
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 Five 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).
Used in the first Roman calendar as well as in the Julian calendar (established by Julius Caesar in 45 B.C.E.) the confusing system of Kalends, Nones, and Ides continued to be used to varying degrees throughout the Middle Ages and into the Renaissance.
So, the Ides of March is just one of a dozen Ides that occur every month of the year. Kalends, the word from which calendar is derived, is another exotic-sounding term with a mundane meaning. Kalendrium means account book in Latin: Kalend, the first of the month, was in Roman times as it is now, the date on which bills are due.