IDES, in the ancient Roman calendar, were eight days in each month; the first of which fell on the 15th of March, May, July, and October; and on the 13th day of the other months.
They were reckoned backwards, in the manner already explained under the article CALENDS.
Thus they called the 14th day of March, May, July, and October; and the 12th of the other months, the pridie idus, or the day before the ides; the next preceding day, they called the tertio idus; and so on, reckoning always backwards, till they come to the nones. See NONES. This method of reckoning time is still retained in the chancery of Rome, and in the calendar of the breviary.