June 2 is the joint feast of Sts. Marcellinus and Peter, two Roman martyrs of the persecution of Diocletian, c. 304. Marcellinus was a priest, Peter an exorcist; both were beheaded in Rome and buried in the cemetery of the Two Laurels on the Via Labicana. Their cult is ancient: Pope Damasus I (366-384) composed an epitaph for their tomb, in which he says he learned the story of their martyrdom from the executioner himself, who had become a Christian after the event.
Constantine the Great built a basilica over their tomb, in which his mother Helena was later buried. The present church of Santi Marcellino e Pietro, on the Via Labicana, is the successor of the Constantinian basilica. The two martyrs are named in the Roman Canon — the first saints of the Roman Canon after the apostles and the early Popes.
The June 2 feast was kept as an obligatory memorial until the 1969 reform reduced it to an optional memorial. From Rome, where they were martyred and are buried, the June 2 feast is the feast of the priest and the exorcist who died together and were named in the Canon of the Mass.
