For many centuries, May 31st was kept as the feast of an early Roman martyr named Petronilla. Nothing is known about her history for certain, beyond the fact that she was buried in the catacomb of Domitilla, about 1½ miles from the Aurelian Walls down the via Ardeatina, where she is depicted in a fresco of the 4th century. A later and unreliable legend makes her the daughter of St Peter, and it was likely for this reason that Pope St Paul I (757-67) translated her relics to her putative father’s basilica in the Vatican.
In the mid-5th century, a large mausoleum was built next to the left transept of the ancient basilica of St Peter. Six of its eight internal niches later became chapels, with that opposite the door being dedicated to St Petronilla; for a long time, this chapel was under the patronage of the kings of France. In 1498, Cardinal Jean Bilhères de Lagraulas, the French ambassador to the Papal court, commissioned his own funerary monument to be added to the chapel, from a 23-year-old Florentine sculptor named Michelangelo Buonarroti. This is, of course, one of the most loved and admired sculptures in the entire world, the Pietà.
(Photo from Wikimedia Commons by Stanislav Traykov, CC BY-SA 3.0)
The art historian Giorgio Vasari (1511-74) was a personal friend of Michelangelo, and relates the story that when the sculpture was originally unveiled, it was left unsigned. But the artist one day happened to enter the mausoleum and overhear a group of pilgrims from Lombardy admiring the statue, and attributing it to their countryman Cristoforo Solari (1460 ca. – 1527), generally known as “the hunchback from Milan.” He then returned to the church at night, and added his signature to the sash that runs across the Virgin Mary’s chest, in Latin, of course: “Michael Angelus Bonarotus Florentinus faciebat. – Michelangelo Buonarroti, a Florentine, was the maker (of this work.)”
Only 7 years after the sculpture’s completion in 1499, Pope Julius II and the architect Donatello Bramante would begin (though just barely) the process of replacing the ancient basilica of St Peter, then in a terrible condition. After decades of delays, Michelangelo himself would take the project in hand in 1545, at the age of 70, and spend the last 19 years of his life working on the great church which we have today. In the 1570s, his successor as chief architect, Giacomo Vignola, demolished the mausoleum where the Pietà originally stood, in order to make way for the left transept of the vastly larger new basilica. The Pietà now stands in its own chapel at the back of St Peter’s, while St Petronilla has a chapel on the opposite end of the building. The new church is so much larger than the old one that this chapel in the northwest corner stands entirely outside the former footprint of the Constantinian structure.
(The ancient basilica of St Peter; the mausoleum which included the chapel of St Petronilla is the rotunda on the far left.)