Ides

Ides: see calendar.

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ides

(in the Roman calendar) the 15th day in March, May, July, and October and the 13th day of each other month

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Ides

Various

In the ancient Roman calendar, the ides fell on the 15th day of March, May, July, and October, and on the 13th day of the other months. The Roman emperor Julius Caesar was assassinated on the Ides of March in 44 b.c.e., and Shakespeare's famous reference to this day in his play Julius Caesar —"Beware the Ides of March"—is probably the best-known use of the term.
The ancient Romans specified a particular day in the month by relating it to the next calends, ides, or nones. For example, "six days before the Ides of June" meant June 8, since the ides in June fell on the 13th.
Calends, sometimes spelled "kalends," refers to the first day of the month, from which the days of the preceding months were counted backward. The order of the days in each month were publicly proclaimed on the calends. For example, "the sixth of the calends of April" meant March 27, or the sixth day before the first day of April (counting April 1 as the first day.)
The Greeks didn't use the term, which is why the phrase "on (or at) the Greek calends" is a synonym for "never." Occasionally, calends was used to mean Settlement Day, since the first of the month was usually the day on which debts were settled.
The nones fell on the ninth day before the ides. In March, May, July, and October, the nones occurred on the seventh of the month because the ides fell on the 15th. In all the other months, the nones occurred on the fifth or 13th days.

SOURCES:
AnnivHol-2000, p. 44
DictDays-1988, p. 18
DictRomRel-1996, p. 104
FestRom-1981, p. 42
OxYear-1999, p. 118