calendaryoudaoicibaDictYouDict[calendar 词源字典]
calendar: [13] English acquired calendar via Anglo-Norman calender and Old French calendier from Latin calendārium, which was a ‘moneylender’s account book’. It got its name from the calends (Latin calendae), the first day of the Roman month, when debts fell due. Latin calendae in turn came from a base *kal- ‘call, proclaim’, the underlying notion being that in ancient Rome the order of days was publicly announced at the beginning of the month.

The calendula [19], a plant of the daisy family, gets its name from Latin calendae, perhaps owing to its once having been used for curing menstrual disorders. Calender ‘press cloth or paper between rollers’ [15], however, has no connection with calendar; it probably comes from Greek kúlindros ‘roller’, source of English cylinder.
[calendar etymology, calendar origin, 英语词源]
calendar (n.)youdaoicibaDictYouDict
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."