IDES, in the Roman calendar, the 15th day of March, May, July, and October, and the 13th day of the other months. The eight days preceding the ides were named from it, and styled the 1st, 2d, 3d, &c., day before the ides. Under the empire the senate sat regularly on the ides and on the calends, with the exception of the ides of March, the anniversary of Cæsar's death, which was regarded as a dies ater.