noun Often with initial capital: the first day of a month.
The third day before the calends of February is 30 January, the third calends of March is 27 or 28 February, and the third of the calends of May is 29 April.
noun The first day of a month of the Roman calendar.
noun A day for settling debts and other accounts.
noun Synonym of Rosh Hodesh (“the Jewish festival of the new moon, which begins the months of the Hebrew calendar”).