Īdus (often eidus, v. Inscr. Orell. 42), ŭum, f. [acc. to Macr. S. 1, 15, from the Etrusc. ‡ iduo, to divide; hence, qs. the divided or half month; but prob. Sanscr. root, indh-, idh-, to kindle, lighten; indu, moon; prop. the days of light, of the moon],
one of the three days in each month from which the other days were reckoned in the Roman calendar , the Ides; it fell upon the fifteenth day of March , May , July , and October; upon the thirteenth day in the remaining months (cf.: Kalendae, Nonae): res ante idus acta sic est: nam haec idibus mane scripsi, Cic. Fam. 1, 1, 3: duas epistulas accepi postridie idus, alteram eo die datam, alteram idibus, id. Att. 15, 17, 1: haec S. C. perscribuntur a. d. VIII. idus Januarias, Caes. B. C. 1, 5, 4: omnia licet concurrant: idus Martiae consolantur, Cic. Att. 14, 4, 2; cf.: stulta jam iduum Martiarum est consolatio, id. ib. 15, 4, 2: si quid vellent, a. d. idus Apr. reverterentur, Caes. B. G. 1, 7 fin. : iduum Septembrium dies, Tac. A. 2, 32: postero iduum dierum, id. H. 1, 26.—The ides were sacred to Jupiter, Varr. ap. Macr. S. 1, 14; cf. idulis.—Interest was paid on the ides: fenerator Alphius, Jam jam futurus rusticus, Omnem redegit idibus pecuniam, Quaerit Kalendis ponere, Hor. Epod. 2, 69: diem pecuniae Idus Novembres esse, Cic. Att. 10, 5, 3: jam vel sibi habeat nummos, modo numeret Idibus, id. ib. 14, 20, 2: praetermitto ruinas fortunarum tuarum, quas omnes impendere tibi proximis Idibus senties, id. Cat. 1, 6, 14.—The payment of school-money at the ides is referred to in: (pueri) Ibant octonis referentes idibus aera, Hor. S. 1, 6, 75; v. Orell. ed h. 1.