The calends or kalends (Latin: kalendae) is the first day of every month in the Roman calendar.
The Romans called the first day of every month the calends, signifying the start of a new lunar phase.
[1] The following lines of poetry aid calculations relating to the day of the month from the calends: Principium mensis cujusque vocato kalendas: Sex Maius nonas, October, Julius, et Mars; Quattuor at reliqui: dabit idus quidlibet octo.
This phrase survived for many centuries in Greek (Ancient Greek: εἰς τὰς ἑλληνικὰς καλένδας) and in the Romance languages (Spanish: hasta las calendas griegas; Italian: alle calende greche; French: aux calendes grecques; Romanian: la calendele grecești; Portuguese: às calendas gregas ; etc.).
It is sometimes claimed that the kalends was frequently used in formal or high-register contexts, and that that is why it retained its traditional spelling, but there seems to be no source for this.