Saint Placidus (also known as Saint Placid) was a disciple of Saint Benedict. He was the son of the patrician Tertullus, was brought as a child to St. Benedict at Sublaqueum (Subiaco) and dedicated to God as provided for in chapter 69 of the Rule of St. Benedict (oblate).
Here too occurred the incident related by St. Gregory the Great (Dialogues, II, vii) of his rescue from drowning when his fellow monk, Saint Maurus, at Saint Benedict's order ran across the surface of the lake below the monastery and drew Placidus safely to shore. It appears certain that he accompanied Saint Benedict when, about 529, he removed to Monte Cassino, which was said to have been made over to him by the father of Placidus.
Of his later life nothing is known, but in an ancient psalterium at Vallombrosa his name is found in the Litany of the Saints placed among the confessors immediately after those of Saint Benedict and Saint Maurus; the same occurs in Codex CLV at Subiaco, attributed to the ninth century.
Saint Placidus (Placitus), along with Saints Eutychius (Euticius), Victorinus and their sister Flavia, Donatus, Firmatus the deacon, Faustus, and thirty others, have been venerated as Christian martyrs. They were said to be martyred either by pirates at Messina or under the Emperor Diocletian.
In their "Acts," this Placidus was confused with a saint of the same name who was a follower of St. Benedict. Thus, the legend of this unknown Sicilian martyr has him go to Italy in 541, and found a monastery at Messina, of which he was abbot, and where he was said to have been martyred with thirty companions.
The feast day of the martyr saints was not in the Tridentine Calendar, but was included in the General Roman Calendar from its 1588 to 1962 editions for celebration on 5 October, the feast day of the two monks who were disciples of Saint Benedict of Nursia from their boyhood, Saint Maurus and Placidus. Some traditionalist Catholics continue to observe pre-1970 calendars.