Middle English kalendis "first day of the month," borrowed from Latin kalendae, calendae, plural noun, probably from an irregularly formed gerundive of calāre "to announce, proclaim" — more at low entry 1

Note: Latin kalendae was one of a small number of words in which an earlier spelling with k was retained in later republican and imperial Rome. The origin of kalendae was thus explained by M. Terentius varro in De lingua latina: "Primi dies mensium nominati Kalendae, quod his diebus calantur eius mensis Nonae a pontificibus, quintanae an septimanae sint futurae, in Capitolio in Curia Calabra sic: 'Die te quinti kalo Iuno Covella'…." ("The first days of the months are called Kalendae, because on these days the Nones of that month are announced [calantur] by the pontifexes—whether they would be on the fifth or seventh days—in the Curia Calabra [a sacred area where the new moon was observed] on the Capitoline Hill, in the following way: 'Juno Covella, I announce [calō] you on the fifth day'….") The form kalendae for expected kalandae has been variously explained; it may be a retention of an earlier conjugation *calere or *calēre (which would accord with Umbrian kařetu—see note at low entry 1).