What is Calends?

Legal Definition
The calends or kalends (Latin: kalendae) is the first day of every month in the Roman calendar. The English word Calendar is derived from this word.
-- Wikipedia
Legal Definition
Among the Romans the first day of every month, being spoken of by itself or the very day of the new moon, which usually happen together. And if pridie, the day before, be added to it, then it is the last, day of the foregoing month, as pridie calend. 8epteml). is the last day of August. If any number be placed with it, it signifies that day in the former month which comes so much before the month named, as the tenth calends of October is the 20th day of September; for if one reckons backwards, beginning at October, that 20th day of September makes the 10th day before October. In March, May, July, and October, the calends begin at the sixteenth day, but in other months at the fourteenth; which calends must ever bear the name of the month following, and be numbered backwards from the first day of the said following months. Jacob. See Rives v. Guthrie, 46 N. C. 87.
-- Black's Law Dictionary
Legal Definition
(Reman) The first day of the month.
-- Ballentine's Law Dictionary