Webster's Revised Unabridged Dictionary · 1913

Ides

Ides (īdz) , noun, plural

[Latin idus: compare French 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. — Shakespeare

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 eighth from the ides.