Etymology
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May Day 

"first of May," on which the opening of the season of flowers and fruit formerly was celebrated throughout Europe, mid-13c.; see May + day (n.). May Queen "girl or young woman crowned with flowers and honored as queen at the games held on May Day," seems to be a Victorian re-invented tradition; the phrase Queen of Maij is attested from c. 1500.

May Day's association with communism (and socialism and anarchism) dates to 1890. A U.S. general strike for an eight-hour workday began May 1, 1886, and culminated in the Haymarket bombing affair in Chicago on May 4. By 1890 strikes, protests, and rallies were being held in Europe by socialist and labor organizations on May 1, at first in support of the eight-hour day, more or less in commemoration of the 1886 strike.

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may (v.2)

"to celebrate May Day, to take part in May Day festivities," late 15c., from May. Maying as "the observance of May Day with all its sports and games" is attested from late 14c. (maiing).

And as a vapour, or a drop of raine
Once lost, can ne'r be found againe:
                     So when or you or I are made
                     A fable, song, or fleeting shade;
                     All love, all liking, all delight
                     Lies drown'd with us in endlesse night.
Then while time serves, and we are but decaying;
Come, my Corinna, come, let's goe a Maying.
[Robert Herrick, "Corinna's Going a-Maying," 1648] 
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may (v.1)

Old English mæg "am able" (infinitive magan, past tense meahte, mihte), from Proto-Germanic root *mag-, infinitive *maganan (Old Frisian mei/muga/machte "have power, may;" Old Saxon mag/mugan/mahte; Middle Dutch mach/moghen/mohte; Dutch mag/mogen/mocht; Old High German mag/magan/mahta; German mag/mögen/mochte; Old Norse ma/mega/matte; Gothic mag/magan/mahte "to be able"), from PIE root *magh- "to be able, have power." A present-preterit verb (with can, shall, etc.). Also used in Old English as a "auxiliary of prediction."

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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 (n.)

Old English dæg "period during which the sun is above the horizon," also "lifetime, definite time of existence," from Proto-Germanic *dages- "day" (source also of Old Saxon, Middle Dutch, Dutch dag, Old Frisian di, dei, Old High German tag, German Tag, Old Norse dagr, Gothic dags), according to Watkins, from PIE root *agh- "a day."  He adds that the Germanic initial d- is "of obscure origin." But Boutkan says it is from PIE root *dhegh- "to burn" (see fever). Not considered to be related to Latin dies (which is from PIE root *dyeu- "to shine").

Meaning originally, in English, "the daylight hours;" it expanded to mean "the 24-hour period" in late Anglo-Saxon times. The day formerly began at sunset, hence Old English Wodnesniht was what we would call "Tuesday night." Names of the weekdays were not regularly capitalized in English until 17c.

From late 12c. as "a time period as distinguished from other time periods." Day-by-day "daily" is from late 14c.; all day "all the time" is from late 14c.  Day off "day away from work" is attested from 1883; day-tripper first recorded 1897. The days in nowadays, etc. is a relic of the Old English and Middle English use of the adverbial genitive.

All in a day's work "something unusual taken as routine" is by 1820. The nostalgic those were the days is attested by 1907. That'll be the day, expressing mild doubt following some boast or claim, is by 1941. To call it a day "stop working" is by 1919; earlier call it a half-day (1838). One of these days "at some day in the near future" is from late 15c. One of those days "a day of misfortune" is by 1936.

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maypole (n.)

also May-pole, "high striped pole decorated with flowers and ribbons for May Day merrymakers to dance around," attested from 1550s but certainly much older, as the first mention of it is in an ordinance banning them, and there are references to such erections, though not by this name, from a mid-14c. Welsh poem. See May Day.

It was usually cut and set up afresh on May-day morning, drawn by a long procession of oxen, decorated, as were also the pole itself and the wagon, with flowers and ribbons; but in some cases a pole once set up was left from year to year, as notably the famous pole of the parish of St. Andrew Undershaft in London, which was cut down in in reign of Edward VI. [Century Dictionary]
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may-fly (n.)

also mayfly, popular name of ephemeral insects of early spring, 1650s, from May + fly (n.).

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good-day (n.)
early 12c., "a fortunate day," also, generally, "good fortune;" from good (adj.) + day (n.). As a salutation in parting, haue godne day "have good day" is recorded from c. 1200; good day as a greeting is from late 14c.
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VJ day (n.)
also V-J Day, "Victory in Japan Day," 1944; it shares an origin with VE Day.
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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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