Dictionary.net

(Calends.) A register or list of things, as a C. of State papers.

A book or table containing the order and sequence of all the days of the year; an almanac; an Ephemeris [Gr.]. In the Julian C. the year is - 365 days; but every fourth year has an additional day, = 366 days. In the Gregorian or Reformed C. , three of these additional days are omitted in the course of 400 years; so that only 97 years in the 400 are 366 days long. The rule is that the year consists of 366 days when its number is divisible by 4, as A.D. 1880, 1884, etc.; but it consists of 365 days when its number, though divisible by 4, consists exactly of centuries and is not divisible by 400; thus, A.D. 1900 will have only 365 days, but A.D. 2000 will have 366 days.