Etymology
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May 

fifth month of the modern calendar, early 12c., Mai, from Old French mai and directly from Latin Majus, Maius mensis "month of May," possibly from Maja, Maia, a Roman earth goddess (wife of Vulcan) whose name is of unknown origin; possibly from PIE *mag-ya "she who is great," fem. suffixed form of root *meg- "great" (cognate with Latin magnus).

"[R]eckoned on the continent of Europe and in America as the last month of spring, but in Great Britain as the first of summer" [Century Dictionary, 1897]. Replaced Old English þrimilce, month in which cows can be milked three times a day. May marriages have been considered unlucky at least since Ovid's day. May-apple, perennial herb native to North America, so called for its time of blooming and its yellowish fruit, is attested from 1733, American English.

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Day-Glo 

1951, proprietary name (Dane & Co. of London) for a brand of fluorescent paint. As an adjective, with reference to colors and patterns, by 1959.

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Earth Day 

as an annual ecological awareness event on April 22, from 1970; the idea for it and the name date from 1969.

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Sunday (n.)

first day of the week, Old English sunnandæg (Northumbrian sunnadæg), literally "day of the sun," from sunnan, oblique case of sunne "sun" (see sun (n.)) + dæg "day" (see day). A Germanic loan-translation of Latin dies solis "day of the sun," which is itself a loan-translation of Greek hēmera heliou. Compare Old Saxon sunnun dag, Old Frisian sunnandei, Old Norse sunnundagr, Dutch zondag, German Sonntag "Sunday."

In European Christian cultures outside Germanic often with a name meaning "the Lord's Day" (Latin Dominica). Sunday-school dates from 1783 (originally for secular instruction); Sunday clothes is from 1640s. Sunday driver is from 1925.

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Decameron (n.)

c. 1600, from Italian Decamerone, titleof Boccaccio's 14c. collection of 100 tales supposedly told over 10 days, from Greek deka "ten" (from PIE root *dekm- "ten") + hēmera "day," from PIE *Hehmer "day" (source also of Armenian awr "day"). Related: Decameronic.

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Monday (n.)

second day of the week, Middle English monedai, from Old English mōndæg, contraction of mōnandæg "Monday," literally "day of the moon," from mona (genitive monan; see moon (n.)) + dæg (see day). A common Germanic name (compare Old Norse manandagr, Old Frisian monendei, Dutch maandag, German Montag). All are loan-translations of Late Latin Lunæ dies, which also is the source of the day name in Romance languages (French lundi, Italian lunedi, Spanish lunes), itself a loan-translation of Greek Selēnēs hēmera. The name for this day in Slavic tongues generally means "day after Sunday."

Yf cristemas day on A munday be,
Grete wynter þat yere ye shull see.
[proverb, c. 1500]

Phrase Monday morning quarterback is attested from 1932, Monday being the first day back at work after the weekend, where school and college football games played over the weekend were discussed. Black Monday (late 14c.) is the Monday after Easter day, though how it got its reputation for bad luck is a mystery (none of the usual explanation stories holds water). Saint Monday (1753) was "used with reference to the practice among workmen of being idle Monday, as a consequence of drunkenness on the Sunday" before [OED]. Clergymen, meanwhile, when indisposed complained of feeling Mondayish (1804) in reference to effects of Sunday's labors.

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Saturday (n.)

seventh or last day of the week, Old English sæterdæg, sæternesdæg, literally "day of the planet Saturn," from Sæternes (genitive of Sætern; see Saturn) + Old English dæg (see day). Partial loan-translation of Latin Saturni dies "Saturn's day" (compare Dutch Zaterdag, Old Frisian Saterdi, Middle Low German Satersdach; Irish dia Sathuirn, Welsh dydd Sadwrn). The Latin word itself is a loan-translation of Greek kronou hēmera, literally "the day of Cronus."

German Samstag (Old High German sambaztag) appears to be from a Greek *sambaton, a nasalized colloquial form of sabbaton "sabbath" (see Sabbath), which also yielded Old Church Slavonic sabota, Polish sobota, Russian subbota, Hungarian szombat, French samedi.

Unlike other English day names there was no Germanic substitution, perhaps because the northern European pantheon lacks a match to Roman Saturn. A homely ancient Nordic custom seems to be preserved in Old Norse day names laugardagr, Danish lørdag, Swedish lördag "Saturday," literally "bath day" (Old Norse laug "bath").

Saturday night has been figurative of revelry and especially "drunkenness and  looseness in relations between the young men and young women" ["Clara Hopwood"] at least since 1847. Saturday-night special "cheap, low-caliber handgun" is American English, attested from 1976 (earlier Saturday-night pistol, 1929).

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Tammany 
in 19c. American English political jargon synonymous with "Democratic Party in New York City," hence, late 19c., proverbial for "political and municipal corruption," from Tammany Hall, on 14th Street, headquarters of a social club incorporated 1789, named for Delaware Indian chief Tamanen, who sold land to William Penn in 1683 and '97. Around the time of the American Revolution he was popularly canonized as St. Tammany and taken as the "patron saint" of Pennsylvania and neighboring colonies, sometimes of the whole of America. He was assigned a feast day (May 1 Old Style, May 12 New Style) which was celebrated with festivities that raised money for charity, hence the easy transfer of the name to what was, at first, a benevolent association. The club's symbol was a tiger.
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Friday (n.)

sixth day of the week, Old English frigedæg "Friday, Frigga's day," from Frige, genitive of *Frigu (see Frigg), Germanic goddess of married love. The day name is a West Germanic translation of Latin dies Veneris "day of (the planet) Venus," which itself translated Greek Aphrodites hēmera.

Compare Old Norse frijadagr, Old Frisian frigendei, Middle Dutch vridach, Dutch vrijdag, German Freitag "Friday," and the Latin-derived cognates Old French vendresdi, French vendredi, Spanish viernes. In Germanic religion, Freya (q.v.) corresponds more closely in character to Venus than Frigg does, and some early Icelandic writers used Freyjudagr for "Friday."

A fast-day in the Church, hence Friday face (17c.) for a gloomy countenance. Black Friday as the name for the busy shopping day after U.S. Thanksgiving holiday is said to date from 1960s and perhaps was coined by those who had the job of controlling the crowds, not by the merchants; earlier it was used principally of Fridays when financial markets crashed (1866, 1869, 1873).

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Barnabas 
surname of Joseph the Levite of Cyprus (Acts iv.36), literally "son of exhortation," from Aramaic (Semitic) bar "son" + nabha "prophecy, exhortation." St. Barnabas' Day (colloquially St. Barnaby), June 11, in the Old Style calendar was reckoned the longest day of the year (Barnaby the Bright).
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