January. So called from “Janus,” the Roman deity that kept the gates of heaven. The image of Janus is represented with two faces looking opposite ways. One face is old, and is emblematical of time past; the other is young, as the emblem of time future. The Dutch used to call this month Lauw-maand (frosty-month); the Saxons, Wulf-monath, because wolves were very trouble-some then from the great scarcity of food. After the introduction of Christianity, the name was changed to Se æftera geóla (the after-yule); it was also called Forma-monath (first month). In the French Republican calendar it was called Nivôse (snow-month, December 20th to 20th January).
February. So called from “Februa,” a name of Juno, from the Sabine word februo (to purify). Juno was so called because she presided over the purification of women, which took place in this month. The Dutch used to term the month Spokkel-maand (vegetation-month); the ancient Saxons, Sprote-cál (from the sprouting of pot-wort or kele); they changed it subsequently to Solmonath (from the returning sun). In the French Republican calendar it was called Pluviôse (rain-month, 20th January to 20th February).
March. So called from “Mars,” the Roman war-god and patron deity. The old Dutch name for it was Lent-maand (lengthening-month), because the days sensibly lengthen; the old Saxon name was Hréth-monath (rough month, from its boisterous winds); the name was subsequently changed to Length-monath (lengthening month); it was also called Hlyd-monath (boisterous-month). In the French Republican calendar it was called Ventôse (windy-month, February 20th to March 20th).
April. So called from the Latin aperio (to open), in allusion to the unfolding of the leaves. The old Dutch name was Gras-maand (grass-month); the old Saxon, Easter-monath (orient or paschal-month). In the French Republican calendar it was called Germinal (the time of budding, March 21st to the 19th of April).
May is the old Latin magius, softened into maius, similar to the Sanskrit mah (to grow), that is, the growing-month. The old Dutch name was Blou-maand (blossoming month); the Old Saxon, Trimilchi (three milch), because cows were milked thrice a day in this month. In the French Republican calendar the month was called Floréal (the time of flowers, April 20th to May 20th).
June. So called from the “juniores” or soldiers of the state, not from Juno, the queen-goddess. The old Dutch name was Zomer-maand (summer-month); the old Saxon, Sere-monath (dry-month), and Lida-arra (joy-time). In the French Republican calendar the month was called Prairial (meadow-month, May 20th to June 18th).
July. Mark Antony gave this month the name of Julius, from Julius Caesar, who was born in it. It had been previously called Quintilis (fifth-month). The old Dutch name for it was Hooymaand (hay-month); the old Saxon, Mæd-monath (because the cattle were turned into the meadows to feed), and Lida æftevr (the second mild or genial month). In the French Republican calendar it was called Messidor (harvest-month, June 19th to July 18th).
August. So called in honour of Augustus Cæsar; not because it was his birth-month, but because it was the month in which he entered upon his first consulship, celebrated three triumphs, received the oath of allegiance from the legions which occupied the Janiculum, reduced Egypt, and put an end to the civil wars. He was born in September. The old Dutch name for August was Oostmaand (harvest-month); the old Saxon, Weod-monath (weed-month, where weed signifies vegetation in general. In the French Republican calendar it was called Ther-midor (hot-month, July 19th to August 17th).
September. The seventh month from March, where the year used to commence. The old Dutch name was Herstmaand (autumn-month); the old Saxon, Gerst-monath (barley-monath), or Hærfest-monath; and after the introduction of Christianity Halig-monath (holy-month, the nativity of the Virgin Mary being on the 8th, the exaltation of the Cross on the 14th, Holy-Rood Day on the 26th, and St. Michael's Day on the 29th). In the French Republican calendar it was called Fructidor (fruit-month, August 18th to September 21st).
October. The eighth month of the Alban calendar. The old Dutch name was Wyn-maand; the Old Saxon, Win-monath (wine-month, or the time of vintage); it was also called Teo-monath (tenth-month), and Winter-fylleth (winter full-moon). In the French Republican calendar it was called Vendé-miaire (time of vintage, September 22nd to October 21st).
November. The ninth Alban month. The old Dutch name was Slaght-maand (slaughter-month, the time when the beasts were slain and salted down for winter use); the old Saxon, Wind-monath (wind-month, when the fishermen drew their boats ashore, and gave over fishing till the next spring); it was also called Blot-monath—the same as Slaght-maand. In the French Republican calendar it was called Brumaire (fog-month, October 22nd to November 21st).
December. The tenth month of the old Alban calendar. The old Dutch name was Winter-maand (winder-month); the old Saxon, Mid-winter-monath (mid-winter-month); whereas June was Mid-sumor-monath. Christian Saxons called December Se ura geóla (the anti-yule). In the French Republican calendar it was called Frimaire (hoar-frost month, from November 22nd to December 20th). Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894