calendar (n.) Look up calendar at
c. 1200, "system of division of the year;" mid-14c. as "table showing divisions of the year;" from Old French calendier "list, register," from Latin calendarium "account book," from calendae/kalendae "calends" the first day of the Roman month -- when debts fell due and accounts were reckoned.

This is from calare "to announce solemnly, call out," as the priests did in proclaiming the new moon that marked the calends, from PIE root kele- (2) "to call, shout" (see claim (v.)). In Rome, new moons were not calculated mathematically but rather observed by the priests from the Capitol; when they saw it, they would "declare" the number of days till the nones (five or seven, depending on the month). The word was taken by the early Church for its register list of saints and their feast days.

Sometimes used in a general sense, "to insert between others" (1824). The -ar spelling in English is 17c. to differentiate it from the now obscure calender "cloth-presser."