The Saints of the Day for May 12 are Saints Nereus, Achilleus and Pancras.
Nereus and Achilleus were Roman soldiers in the household of Flavia Domitilla. They were instructed and converted by St. Peter. These two soldiers in turn inspired St. Domitilla to consecrate her virginity to God. Thereupon, Aurelianus, the fiancee of Domitilla, reported all three to the Roman authorities as being Christians. They were martyred out of hatred for Christianity. Pancras, a noble Phrygian youth, came to Rome at the age of fourteen, and was martyred in 275 because he refused to offer sacrifices to the pagan gods.
It was under the persecution of Domitian, during which John the Evangelist was condemned to be burned alive in the cauldron of boiling oil, that Flavia Domitilla was honored with banishment and death for the sake of our Redeemer, whom she had chosen for her Spouse. She was of the imperial family, being a niece of Flavius Clemens, who adorned the consular dignity by martyrdom. She was one of the Christians belonging to the court of the Emperor Domitian, who show us how rapidly the religion of the poor and humble made its way to the highest classes of Roman life. A few years previous to this, St Paul sent to the Christians of Philippi the greetings of the Christians of Nero's palace. There is still extant, not far from Rome, on the Ardeatine Way, the magnificent subterranean cemetery which Flavia Domitilla ordered to be dug on her praedium, and in which were buried the two martyrs, Nereus and Achilleus, whom the Church honors today together with the noble virgin who owes her crown to them. Nereus and Achilleus were in Domitilla's service. Hearing them one day speaking of the merit of virginity, she there and then bade farewell to all worldly pleasures, and aspired to the honor of being the Spouse of Christ. She received the veil of consecrated virgins from the hands of Pope St Clement: Nereus and Achilleus had been baptized by St Peter himself.
The bodies of these three Saints reposed, for several centuries, in the Basilica, called the Fasciola, on the Appian Way; and we have a Homily which St Gregory the Great preached in this Church on their feast. The holy Pontiff dwelt on the vanity of the earth's goods; he encouraged his audience to despise them by the example of the three martyrs whose relics lay under the very altar around which they were that day assembled. "These Saints," said he, "before whose tomb we are now standing, trampled with contempt of soul on the world and its flowers. Life was then long, health was uninterrupted, riches were abundant, parents were blessed with many children; and yet, though the world was so flourishing in itself, it had long been a withered thing in their hearts."
— Excerpted from The Liturgical Year, Abbot Gueranger O.S.B.