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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 -- 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.)).
Taken by the early Church for its register list of saints and their feast days. The -ar spelling in English is 17c. to differentiate it from the now obscure calender "cloth-presser."