Latium
The land of the Latins around Rome, where, Varro held, the month names were in use before Rome was founded.
Today: Lazio, Italy
How the Romans fixed the year
Censorinus, On the Birthday 20–22 (238 CE), in the edition of Friedrich Hultsch (Leipzig, 1867) · 238 CE
Varro autem Romanos a Latinis nomina mensum accepisse arbitratus auctores eorum antiquiores quam urbem fuisse satis argute docet.
But Varro held that the Romans took the names of the months from the Latins. He shows quite cleverly that those who made the names were older than the city.
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.
May, he says, took its name not from the elders but from Maia, because in this month sacrifice is made to Maia and Mercury, now at Rome as before in Latium. June too came from Juno rather than from the younger men, because in that month Juno is honored most of all.
Quintilem, quod loco iam apud Latinos 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.
Quintilis was so called because it was already fifth in place among the Latins, and likewise Sextilis and the rest up to December were named from numbers. January and February, he says, were indeed added later, but with names already taken from Latium. January took its name from Janus, to whom it is assigned, and February from februum.
Where the months got their names
Macrobius, Saturnalia 1.12–14 (early fifth century CE), in the edition of Franz Eyssenhardt (Leipzig, 1893) · early fifth century CE; the dialogue is set in the 380s
Iunius Maium sequitur aut ex parte populi, ut supra diximus, nominatus aut, ut Cingius arbitratur, quod Iunonius apud Latinos ante uocitatus, diuque apud Aricinos Praenestinosque hac appelatione in fastos relatus sit adeo ut, sicut Nisus in commentariis fastorum dicit apud maiores quoque nostros haec appelatio mensis diu manserit, sed post detritis quibusdam litteris ex Iunonio Iunius dictus sit. nam et aedes Iunoni Monetae Kalend. Iuniis dedicata est.
June follows May. It was named either after a part of the people, as I said above, or, as Cingius judges, because the Latins once called it Iunonius [after Juno]. The people of Aricia and Praeneste long kept this name in their calendars. As Nisus says in his notes on the calendar, our ancestors too kept this name of the month for a long time, but later some letters wore away and Iunonius became Iunius. Indeed, the temple of Juno Moneta was dedicated on the first of June.