Calends (Lat. Kalendæ), the first day of each Roman month, which was divided into Calends, Nones, and Ides. The Calends always fell upon the first of the month; in March, May, July, and October, the Nones on the 7th, and the Ides on the 15th; and in the remaining months, the Nones on the 5th, and the Ides on the 13th. The Calends meant originally the day on which the order of days was proclaimed; the Nones were so called from being the ninth day before the Ides, reckoning inclusively; and the Ides probably from an Etruscan verb signifying to divide, because they nearly halved the month. This threefold division also determined the reckoning of the days, which were not distinguished as with us by the ordinal numbers first, second, third, &c., but as follows: Those between the Calends and the Nones were termed the days before the Nones; those between the Nones and the Ides, the days before the Ides; and the remainder, the days before the Calends of the next month. Thus, the Ides of January happening on the 13th of that month, the next day would not be termed by a Latin writer the 14th, but the 19th before the Calends of February, reckoning inclusively—i.e. reckoning both the 14th of January and the 1st of February, and so on to the last, which was termed pridie Calendas.
Ad Kalendas Græcas was a Roman proverbial saying, practically equivalent to 'never.' The Roman Calends were the usual days for payment of rent, interest, &c.; but as the Greeks did not use that mode of reckoning, a postponement of payment 'to the Greek Calends' simply meant a refusal to pay altogether. It is said that the Emperor Augustus frequently used the phrase, which afterwards became a proverb.