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 the 13th day of the other months of the year. The origin of the word is contested. Some suppose it to have been formed from idus, to see, because the full moon was commonly seen on the days of the ides; others derive it from ides, species, figure, on account of the image of the full moon then visible; others from idium or ovis iditis, a name given by the Etruscans to a victim offered on that day to Jupiter; and others again from the Etruscan word iduo, meaning to divide, because the ides divided the month into two nearly equal parts.

The ides came between the kalends and the nones, and were reckoned backwards. 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 tercia idus; and so on, reckoning always backwards till they came to the nones. This method of reckoning time is still retained in the chancery of Rome, and in the calendar of the Breviary.

The ides of May were consecrated to Mercury; and the ides of March were ever esteemed unfortunate, after Caesar's murder on that day. The time after the ides of June was reckoned fortunate for those who entered into matrimony; the ides of August were consecrated to Diana, and were observed as a feast-day by the slaves. On the ides of September, auguries were taken for appointing the magistrates, who formerly entered into their offices on the ides of May, and afterwards on those of March.