Etymology
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calendar (n.)
c. 1200, "the year as divided systematically into days and months;" mid-14c. as "table showing divisions of the year;" from Old French calendier "list, register," from Latin calendarium "account book," from calendae/kalendae "the 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 shout." 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. Meaning "list of documents arranged chronologically" is from late 15c.

The -ar spelling in English is 17c. to differentiate it from the now-obscure calender "cloth-presser." Related: Calendarial; calendary.
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calends (n.)
c. 1200, "a day as reckoned back from the first of the following month" (as fourteenth calend of March = February 16th), from Latin kalendae "first day of the month" in the Roman calendar (see calendar). From mid-14c. as "the first day of the month;" late 14c. as the beginning of anything.
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intercalate (v.)
"to insert a day into the calendar," 1610s, from Latin intercalatus, past participle of intercalare "to proclaim the insertion of an intercalary day," from inter "between" (see inter-) + calare "to call" (an intercalary day; see calendar). Sometimes used in a general sense, "to insert between others" (1824). Related: Intercalated; intercalating.

A necessary process in the Roman calendar to balance the solar and lunar aspects of it. Intercalation was done after Feb. 23 or 24 (the terminalia), every two or four years. Twenty-seven days were intercalated, making a full intercalary month (which included the last four or five days of Februarius), known as mensis intercalaris (and also known, according to Plutarch, as Mercedonius). No one now knows why the intercalation was done in the middle of February rather than after its end, unless it was because the important festivals at the end of that month (Regifugium and Equirra) were closely associated with holidays in early March. After Caesar's reform (46 B.C.E.) the only intercalary day is Feb. 29 every four years.
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*kele- (2)
*kelə-, Proto-Indo-European root meaning "to shout." Perhaps imitative.

It forms all or part of: acclaim; acclamation; Aufklarung; calendar; chiaroscuro; claim; Claire; clairvoyance; clairvoyant; clamor; Clara; claret; clarify; clarinet; clarion; clarity; class; clear; cledonism; conciliate; conciliation; council; declaim; declare; disclaim; ecclesiastic; eclair; exclaim; glair; hale (v.); halyard; intercalate; haul; keelhaul; low (v.); nomenclature; paraclete; proclaim; reclaim; reconcile.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit usakala "cock," literally "dawn-calling;" Latin calare "to announce solemnly, call out," clamare "to cry out, shout, proclaim;" Middle Irish cailech "cock;" Greek kalein "to call," kelados "noise," kledon "report, fame;" Old High German halan "to call;" Old English hlowan "to low, make a noise like a cow;" Lithuanian kalba "language."
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intercalary (adj.)
"inserted into the calendar," 1610s, from Latin intercalarius "intercalary, of an intercalary month," from intercalare "proclaim an intercalary day" (see intercalate). General sense of "interpolated" is attested from 1798.
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Gregorian (adj.)
"pertaining to Gregory," from Late Latin Gregorianus, from Gregorius (see Gregory). From c. 1600 of church music, in reference to Gregory I the Great (pope from 590-604), who traditionally codified it; 1640s in reference to new calendar (introduced 1582) from Pope Gregory XIII (1572-1585); due to Protestant resistance, the calendar was not introduced in England and the American colonies until 1752.
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September 
late Old English, from Latin September (also source of Old French Septembre, Spanish Setiembre, Italian Settembre, German September), from septem "seven" (see seven). So called because it was the seventh month of the old Roman calendar, which began the year in March; Julian calendar reform (46 B.C.E.) shifted the new year back two months. For -ber suffix, see December. Replaced Old English hærfestmonað, haligmonað. Related: Septembral.
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ephemeris (n.)
table showing predicted positions of heavenly bodies, 1550s, Modern Latin, from Greek ephemeris "diary, journal, calendar," from ephemeros "daily" (see ephemera). The classical plural is ephemerides.
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hegira (n.)
flight of Muhammad from Mecca to Medina (July 16, 622 C.E.), the event from which the Islamic calendar reckons, 1580s, from Medieval Latin hegira, from Arabic hijrah "departure," from hajara "to depart."
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embolism (n.)
late 14c., "intercalation, insertion of days into a calendar," from Old French embolisme "intercalation," from Late Latin embolismus "insertion of days in a calendar to correct errors," from Late Greek embolismos "intercalation," from embolos "peg, stopper; anything pointed so as to be easily thrust in," also "a tongue (of land), beak (of a ship)," from emballein "to insert, throw in, invade" from assimilated form of en "in" (see en- (2)) + ballein "to throw" (from PIE root *gwele- "to throw, reach," with extended sense "to pierce"). Medical sense of "obstruction of a blood vessel" is first recorded in English 1855. Related: embolismic.
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