Ides \Ides\, n. pl. [L. idus: cf. F. ides.]

(Anc. Rom. Calendar) The fifteenth day of March, May, July, and October, and the thirteenth day of the other months.

The ides of March remember. --Shak.

Note: Eight days in each month often pass by this name, but only one strictly receives it, the others being called respectively the day before the ides, and so on, backward, to the eightth from the ides.

Source: Webster's Revised Unabridged Dictionary (1913)