Source : Webster's Revised Unabridged Dictionary (1913)

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 : WordNet®

ides
     n : in the Roman calendar: the 15th of March or May or July or
         October or the 13th of any other month