Etymology
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month (n.)

"one-twelfth part of a year; one of the twelve parts into which the calendar year is arbitrarily divided," Old English monað, from Proto-Germanic *menoth- (source also of Old Saxon manoth, Old Frisian monath, Middle Dutch manet, Dutch maand, Old High German manod, German Monat, Old Norse manaðr, Gothic menoþs "month"), which is related to *menon- "moon" (see moon (n.)). Originally the month was the interval between one new moon and the next (a sense attested from late Old English).

Its cognates mean only "month" in the Romance languages, but in Germanic they generally continue to do double duty. The development of the calendrical meaning for words from this root in Greek (mēn) and Latin (mensis) was accompanied by the creation of new words for "moon" (selēnē, luna). The phrase a month of Sundays "a very long time" is from 1832 (roughly 7 and a half months but never used literally).

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twelve-month (n.)
"a year," Old English twelf-monð; see twelve + month.
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monthly (adv.)

"once a month, in every month," 1530s, from month + -ly (2).

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monthly (adj.)

1570s, "pertaining to or relating to a month," from month + -ly (1). Old English had monaþlic, but the modern words seem to be separate formations. From 1580s as "continued for a month;" 1640s as "done or happening once every month." As a noun, "a magazine or periodical published once a month," by 1833.

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menstrual (adj.)

late 14c., "pertaining to menses of females," from Old French menstruel and directly from Latin menstrualis "monthly," especially "of or having monthly courses," from menstruus "of a month, every month, monthly, pertaining to a month," from mensis "month" (see moon (n.)). Occasionally, in astronomy, "monthly" (1590s).

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ultimo (adv.)
"in the month preceding the present," 1610s, common in abbreviated form ult. in 18c.-19c. correspondence and newspapers, from Latin ultimo (mense) "of last (month)," ablative singular masc. of ultimus "last" (see ultimate). Earlier it was used in the sense of "on the last day of the month specified" (1580s). Contrasted with proximo "in the next (month)," from Latin proximo (mense).
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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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menstruous (adj.)

"having the monthly flow or discharge," early 15c., from Old French menstrueus and directly from Medieval Latin *menstruosus, from Latin menstruum "of or belonging to menstruation," from menstruus (adj.) "that happens every month, monthly," from mensis "month" (see moon (n.)).

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Ramadan (n.)

ninth month of the Muslim year, period of the annual thirty-days' fast, 1590s, earlier Ramazan (c. 1500), from Arabic Ramadan (Turkish and Persian ramazan), originally "the hot month," from ramida "be burnt, scorched" (compare Mishnaic Hebrew remetz "hot ashes, embers"). In the Islamic lunar calendar, it passes through all seasons in a cycle of about 33 years, but evidently originally it was a summer month.

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proximo 

in correspondence, etc., "in or of the next or coming month," noting a day in the coming month (proximo mense), Latin ablative singular of proximus "nearest, next" (see proximate). Often abbreviated prox. Compare ultimo, instant (adj.).

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