ides (n.)

"middle day of a Roman month," early 14c., from Old French ides (12c.), from Latin idus (plural) "the ides," a word perhaps of Etruscan origin. In the Roman calendar the eighth day after the nones, corresponding to the 15th of March, May, July, and October; the 13th of other months. "Debts and interest were often payable on the ides" [Lewis].