The Calends (Latin Kalendae "the called", gen. plural -arum), correspond to the first days of each month of the Roman calendar. The Romans assigned these calends to the first day of the month, signifying the start of the new moon cycle. On that day, the pontiffs would announce at the Curia Calabra the rest days for the upcoming month, and the debtors had to pay off their debts that were inscribed in the calendaria, a sort of accounts book. The date (in this calendar system) was measured relative to days such as the Calends, Nones or Ides, for example, in modern terms, three days past Calends would be the 4th of the month. This sort of system would be used to date documents, diary entries, etc.

The rules for computation by Calends are included in the following verses:

Prima dies mensis cuiusque est dicta Kalendae: sex Maius Nonas, October, Iulius, et Mars; quattuor at reliqui: dabit Idus quilibet octo. Inde dies reliquos omnes dic esse Kalendas; quas retro numerans dices a mense sequente.

After calculating this number, the pontiff would say something like:

Quinque dies te calo, Iuno nouella.

meaning: I invoke you, new Juno, for five days.

To find the day of the Calends we are in, count how many days remain in the month, and to that number add two. For example, April 22nd is the 10th of the Calends of May, because there are 8 days left in April, to which 2 being added, the sum is 10.

This word forms the basis of a collection of words in the English language, notably calendar. The term itself is traditionally written with "K" at the beginning, following the ancient custom to do that in few words beginning with the "ka" syllable.

The calends being a feature of the Latin calendar, the Greek calendar had no calends. Accordingly, to postpone something ad Kalendas Graecas (to the Greek calends) meant postponing it forever. The phrase survived over the centuries in Greek and in the romance languages (French: aux calendes grecques; Portuguese: às calendas gregas; Romanian: la Calendele Grecesti; etc.).

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