Etymology
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August 
eighth month, late 11c., from Latin Augustus (mensis), sixth month of the later Roman calendar, renamed from Sextilis (literally "sixth") in 8 B.C.E. to honor emperor Augustus Caesar, literally "Venerable Caesar" (see august (adj.), and compare Augustus). One of two months given new names to honor Roman leaders (July being the other), the Romans also gave new imperial names to September (Germanicus) and October (Domitian) but these did not stick.

In England, the name replaced native Weodmonað "weed month." Traditionally the first month of autumn in Great Britain, the last of summer in the U.S.
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Thammuz (n.)
1530s, from Hebrew tammuz, tenth month of the Jewish civil year, fourth of the sacred, covering parts of June and July; also the name of a Syrian deity equivalent to Phoenician Adon, whose festival began with the new moon of this month (compare Tammuz).
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February (n.)

month following January, late 14c., ultimately from Latin februarius mensis "month of purification," from februare "to purify," from februa "purifications, expiatory rites" (plural of februum "means of purification, expiatory offerings"), which is of uncertain origin, said to be a Sabine word. De Vaan says from Proto-Italic *f(w)esro-, from a PIE word meaning "the smoking" or "the burning" (thus possibly connected with fume (n.)). The sense then could be either purification by smoke or a burnt offering.

The last month of the ancient (pre-450 B.C.E.) Roman calendar, so named in reference to the Roman feast of purification, held on the ides of the month. The Old English name for it was solmonað, which is said to mean "mud month." English first borrowed the Roman name from Old French Feverier, which yielded Middle English Feverer, Feoverel, etc. (c. 1200) before the 14c. respelling to conform to Latin.

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April 
fourth month, c. 1300, aueril, from Old French avril (11c.), from Latin (mensis) Aprilis, second month of the ancient Roman calendar, from a stem of uncertain origin and meaning, with month-name suffix -ilis as in Quintilis, Sextilis (the old names of July and August).

Perhaps based on Apru, an Etruscan borrowing of Greek Aphrodite. Or perhaps *ap(e)rilis "the following, the next," from its place as the second month of the old Roman calendar, from Proto-Italic *ap(e)ro-, from PIE *apo- "away, off" (see apo-; compare Sanskrit aparah "second," Gothic afar "after"). Old folk etymology connected it with Latin aperire "to open."

In English in Latin form from mid-12c.; it replaced Old English Eastermonað, which was named for a fertility goddess (see Easter). Re-spelled in Middle English on Latin model (apprile, first attested late 14c.).
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July 

seventh month, c. 1050, Iulius, from Anglo-French julie, Old French Juil, Jule (Modern French uses a diminutive, Juillet) and directly from Latin Iulius "fifth month of the Roman calendar" (which began its year in March), renamed after his death and deification in honor of Gaius Julius Caesar, who was born in this month. In republican Rome it had been Quintilis, literally "fifth." Compare August. Accented on the first syllable in English until 18c.; "the modern Eng. pronunciation is abnormal and unexplained" [OED]. Replaced Old English liða se æfterra "later mildness," from liðe "mild."

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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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December (n.)

"twelfth and last (by modern reckoning) month of the calendar, the month of the winter solstice," late Old English, from Old French decembre, from Latin December, from decem "ten" (from PIE root *dekm- "ten"); tenth month of the old Roman calendar, which began with March.

The -ber in four Latin month names is probably from -bris, an adjectival suffix. Tucker thinks that the first five months were named for their positions in the agricultural cycle, and "after the gathering in of the crops, the months were merely numbered."

If the word contains an element related to mensis, we must assume a *decemo-membris (from *-mensris). October must then be by analogy from a false division Sep-tem-ber &c. Perhaps, however, from *de-cem(o)-mr-is, i.e. "forming the tenth part or division," from *mer- ..., while October = *octuo-mr-is. [T.G. Tucker, "Etymological Dictionary of Latin"]

Decembrist, in Russian history in reference to the insurrection against Nicholas I in December 1825, is by 1868 in English, translating Russian dekabrist, from dekabr' "December."

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January (n.)
late 13c., Ieneuer (early 12c. in Anglo-French), from Old North French Genever, Old French Jenvier (Modern French Janvier), from Latin Ianuarius (mensis) "(the month) of Janus" (q.v.), to whom the month was sacred as the beginning of the year according to later Roman reckoning (cognates: Italian Gennaio, Provençal Genovier, Spanish Enero, Portuguese Janeiro). The form was gradually Latinized by c. 1400. Replaced Old English geola se æfterra "Later Yule." In Chaucer, a type-name for an old man.
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March 

third month of our year, first month of the ancient Roman calendar, c. 1200, from Anglo-French marche, Old French marz, from Latin Martius (mensis) "(month) of Mars," from Mars (genitive Martis). The Latin word also is the source of Spanish marzo, Portuguese março, Italian marzo, German März, Dutch Maart, Danish Marts, etc.

Replaced Old English hreðmonaþ, the first part of which is of uncertain meaning, perhaps from hræd "quick, nimble, ready, active, alert, prompt." Another name for it was Lide, Lyde (c.1300), from Old English hlyda, which is perhaps literally "noisy" and related to hlud "loud" (see loud). This fell from general use 14c. but survived into 19c. in dialect.

For March hare, proverbial type of madness, see mad (adj.). The proverb about coming in like a lion and going out like a lamb is since 1630s. March weather has been figurative of changeableness since mid-15c.

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Pyanepsia (n.)

festival in honor of Apollo on the 7th of Pyanepsion (fourth month of the Attic calendar, corresponding to October-November), from Greek Pyanepsia (plural), literally "the feast of cooking beans," from pyanos, variant of kyamos, name of a kind of bean, a word of unknown origin (perhaps foreign or Pre-Greek), + epsein "to boil, cook." At this festival a dish of pulse was offered to the god.

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