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.).