Nomina decem mensibus antiquis Romulum fecisse Fulvius et Iunius auctores sunt: et quidem duos primos a parentibus suis nominasse, Martium a Marte patre, Aprilem ab Aphrodite id est Venere, unde maiores eius oriundi dicebantur; proximos duos a populo: Maium a maioribus natu, Iunium a iunioribus; ceteros ab ordine quo singuli erant: Quintilem usque Decembrum perinde a numero. Varro autem Romanos a Latinis nomina mensum accepisse arbitratus auctores eorum antiquiores, quam urbem, fuisse satis argute docet. itaque Martium mensem a Marte quidem nominatum credit, non quia Romuli fuerit pater, sed quod gens Latina bellicosa; Aprilem autem non ab Aphrodite, sed ab aperiendo, quod tunc ferme cuncta gignantur et nascendi claustra aperiat natura; Maium vero non a maioribus, sed a Maia nomen accepisse, quod eo mense tam Romae, quam antea in Latio res divina Maiae fit et Mercurio; Iunium quoque a Iunone potius, quam iunioribus, quod illo mense maxime Iunoni honores habentur; Quintilem, quod loco iam apud Latinus fuerit quinto, item Sextilem ac deinceps ad Decembrem a numeris appellatos. ceterum Ianuarium et Februarium postea quidem additos, sed nominibus iam ex Latio sumptis: et Ianuarium ab Iano, cui adtributus est, nomen traxisse, Februarium a februo: est februum quidquid piat purgatque, et februamenta purgamenta, item februare purgare et purum facere. februum autem non idem usquequaque dicitur: nam aliter in aliis sacris februatur, hoc est purgatur. in hoc autem mense Lupercalibus, cum Roma lustratur, salem calidum ferunt, quod februum appellant, unde dies Lupercalium proprie februatus et ab eo porro mensis Februarius vocitatur. ex his duodecim mensibus duorum tantum nomina inmutata: nam Quintilis Iulius cognominatus est C. Caesare V et M. Antonio cons. anno Iuliano secundo; qui autem Sextilis fuerat, ex S.C. Marcio Censorino C. Asinio Gallo cons. in Augusti honorem dictus est Augustus anno Augusti vicensimo, quae nomina etiam nunc ad hanc permanent memoriam. postea vero multi principes nomina quaedam mensium inmutaverunt suis nuncupando nominibus: quod aut ipsi postmodum mutaverunt, aut post obitum eorum illa nomina pristina suis reddita mensibus.
(Censorinus, De Die Natali 22.9-17)
If we believe Fulvius and Junius, it was to Romulus that the ten ancient months owed their names. He gave to the first two the names of the authors of his life; he called one March, from Mars, his father, and the second April, from the word Aphrodite, that is to say, Venus, from whom his ancestors were said to have descended. The next two months take their names from classes of the people; May, from Majores (the old people), and June, from Juniores (the young people); the others, that is to say, Quintilis to December, from the numerical rank which each month occupied in the year. Varro, on the contrary, thought that the Romans borrowed the names of their months from the Latins. He demonstrated in quite a plausible manner that these names are older than the city of Rome. Thus, according to him, the month of March was thus named, not because this god was the father of Romulus, but because the Latin nation were warlike and originally worshipped the god of war. He contends that Aprilis (April) does not take its name from Aphrodite, but from the word aperire (to open), because in this month everything comes to life and nature opens its bosom to all productions. May does not come from majores, but from Maia; because it was in this month that at Rome, and formerly in Latium, sacrifices were made to Holy Maia and Mercury. June comes from Juno rather than from juniores; because it is in this month especially that Juno is worshipped. Quintilis is so called because with the Latins it was the fifth month; it was the same with Sextilis and the other months until December, which all take their names from their numerical order in the year. January and February, it is true, have been since added, but their names come from Latium; January from Janus, to whom this month is consecrated; and February from Februus. All that which serves to expiate and purify is called februum, and all expiations or purifications are called februamenta, just as februare signifies to render clear and pure. The ceremony called februm is not always the same, and the king of purification called februation varies according to the sacrifice. During the Lupercales and the purification of the city, ceremonies which took place during this month, hot salt was carried about, called februm. From this it follows that the days of the Lupercales are properly called februatus; and hence, also, this month took the name of February. Of the twelve months, two only have since changed name; the ancient Quintilis was called Julius (July) under the fifth consulate of Caius Cæsar and under that of M. Antonius in the second Julian year; that which was called Sextilis was, after a senatus-consulto, rendered under the consulate of Marcus Censorinus and C. Asinius Gallus, named Augustan (August), in honour of Augustus, in the 20th year of the Augustan æra; and these names are still retained. Some of the successors of Augustus, it is true, imposed their own names on several months, but the old names were restored, either by the princes themselves, or after their deaths. (tr. William Maude)