"the middle of summer, the period of the summer solstice," Old English midsumor, from mid (adj.) + sumor "summer" (see summer (n.1)). Midsummer Day, an English quarter-day, was June 24 (Feast of the Nativity of St. John the Baptist). Astronomically, midsummer falls on June 21, but the event traditionally was reckoned in northern Europe on the night of June 23-24 (with summer beginning at the start of May). Midsummer night was an occasion of superstitious practices and wild festivities.

international radio-telephone distress call, 1923, apparently an Englished spelling of French m'aider, shortening of venez m'aider "come help me!" But possibly a random coinage with coincidental resemblance:

"May Day" Is Airplane SOS

ENGLISH aviators who use radio telephone transmitting sets on their planes, instead of telegraph sets, have been puzzling over the problem of choosing a distress call for transmission by voice. The letters SOS wouldn't do, and just plain "help!" was not liked, and so "May Day" was chosen. This was thought particularly fitting since it sounds very much like the French m'aidez, which means "help me." ["The Wireless Age," June 1923]

"the whole natural day, a day and a night, twenty-four hours," 1680s, from Greek nykhthēmeron "a day and a night," from nykt-, a combining form of nyx "night" (see night) + hēmera "day," from PIE *Hehmer "day."

1510s, "by the day, in each day," Latin, "by the day," from per (see per) + diem, accusative singular of dies "day" (from PIE root *dyeu- "to shine"). As a noun from 1809, "amount or allowance of so much every day."

also pay-day, "day when payment is to be made or debts are to be discharged," 1520s, from pay (n.) + day.

also day-long, "lasting all day," Old English dæglang; see day + long (adj.).

also neo-paganism, "a revival or reproduction of paganism," 1858; see neo- "new" + paganism. Related: Neopagan (1854 as an adjective, 1855 as a noun).

[The 'positive' philosopher of the present day] offers in the stead of Christianity a specious phase of neo-paganism, by which the nineteenth century after Christ may be assimilated to the golden age of Mencius and Confucius; or, in other words, may consummate its intellectual freedom, and attain the highest pinnacle of human progress, by reverting to a state of childhood and of moral imbecility. [Introduction to Charles Hardwick, "Christ and Other Masters," Cambridge, 18758]

late 14c., originally a medical term, from Medieval Latin ephemera (febris) "(fever) lasting a day," from fem. of ephemerus, from Greek ephemeros "daily, for the day," also "lasting or living only one day, short-lived," from epi "on" (see epi-) + hēmerai, dative of hēmera "day," from PIE *Hehmer "day." Sense extended 17c. to short-lived insects (Modern Latin ephemera musca) and flowers; general sense of "thing of transitory existence" is first attested 1751. Compare Greek ephemeroi "men," literally "creatures of a day."

late 14c., "that moves," present-participle adjective from move (v.). From 1650s as "that causes motion;" 1590s as "that touches the feelings." Moving picture in the cinematographic sense is by 1896 (earlier in reference to the zoetrope, 1709). Moving Day is by 1832, American English; traditionally in New York city it was May 1. Moving target (1833) is from gunnery.