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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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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hecatomb (n.)

1590s, from Latinized form of Greek hekatombe, properly (and literally) "offering of 100 oxen," but generally "a great public sacrifice." It is a compound of hekaton "one hundred," which perhaps is dissimilation of *hem-katon, with hen, neuter of heis "one" + *katon "hundred." The second element is bous "ox" (from PIE root *gwou- "ox, bull, cow"). The first month of the Attic calendar (corresponding to July-August) was Hekatombaion, in which sacrifices were made.

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lunacy (n.)
1540s, "condition of being a lunatic," formed irregularly in English from lunatic (q.v.) + -cy. Originally in reference to intermittent periods of insanity, such as were believed to be triggered by the moon's cycle. The Old English equivalent was monaðseocnes "month-sickness." In later legal use, any unsoundness of mind sufficient to render one incapable of civil transactions or management of one's affairs. Weakened figurative sense "act of madness or folly" is from 1580s.
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sere (adj.)
Old English sear "dried up, withered, barren," from Proto-Germanic *sauzas (source also of Middle Low German sor, Dutch zoor "dry"), from PIE root *saus- "dry" (source also of Sanskrit susyati "dries, withers;" Old Persian uška- "dry" (adj.), "land" (n.); Avestan huška- "dry;" Greek auos "dry," auein "to dry;" Latin sudus "dry"). A good word now relegated to bad poetry. Related to sear. Sere month was an old name for "August."
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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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neap (adj.)

"low, lowest," applied to tides which have the least difference of height between the flood and ebb, late 15c., from Old English nepflod "neap flood," the tide occurring at the end of the first and third quarters of the lunar month, in which high waters are at their lowest, of unknown origin, with no known cognates (Danish niptid probably is from English). Original sense perhaps is "without power." As a noun from 1580s, "a neap tide," also sometimes in modern use "the ebb or lowest point of a tide."

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legem pone (n.)
"payment of money, cash down," 1570s, old slang, from the title in the Anglican prayer-book of the psalm appointed for Matins on the 25th of the month; it was consequently associated especially with March 25, the new year of the old calendar and a quarter day, when payments and debts came due and money changed hands generally. The title is from the first two words of the fifth division of Psalm cxix: Legem pone mihi, Domine, viam justificationum tuarum "Teach me, O Lord, the way of thy statutes."
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honeymoon (n.)
"indefinite period of tenderness and pleasure experienced by a newly wed couple," 1540s (hony moone), but probably older, from honey (n.) in reference to the new marriage's sweetness, and moon (n.) "month" in reference to how long it probably will last, or from the changing aspect of the moon: no sooner full than it begins to wane. French has cognate lune de miel, but German version is flitterwochen (plural), from flitter "tinsel" + wochen "week." In figurative use from 1570s. Specific sense of "post-wedding holiday" attested from c. 1800.
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