Mercury
The Roman god of speech, merchants and trade, son of Maia, to whom merchants sacrificed in May.
How the Romans fixed the year
Censorinus, On the Birthday 20–22 (238 CE), in the edition of Friedrich Hultsch (Leipzig, 1867) · 238 CE
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.
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
contendunt alii Maiam Mercurii matrem mensi nomen dedisse, hinc maxime probantes, quod hoc mense mercatores omnes Maiae pariter Mercurioque sacrificant.
Others argue that Maia, the mother of Mercury, gave the month its name. Their main proof is that in this month all merchants sacrifice to Maia and Mercury together.
adfirmant quidam, quibus Cornelius Labeo consentit, hanc Maiam, cui mense Maio res diuina celebratur, terram esse hoc adeptam nomen a magnitudine, sicut et Mater Magna in sacris uocatur, adsertionemque aestimationis suae etiam hinc colligunt quod sus praegnans ei mactatur, quae hostia propria est terrae. et Mercurium ideo illi in sacris adiungi dicunt, quia uox nascenti homini terrae contactu datur: scimus autem Mercurium uocis et sermonis potentem.
Some people claim, and Cornelius Labeo agrees with them, that this Maia, who is worshiped in May, is the Earth. She got the name from her greatness (magnitudo), just as she is also called the Great Mother in our rites. They support their view with another fact: a pregnant sow is sacrificed to her, and that is the offering proper to the Earth. They also say that Mercury is joined with her in worship because a newborn child gets its voice from touching the earth, and we know that Mercury is the lord of voice and speech.