The only historical information which we possess regarding these two martyrs is the discovery of their bodies by St. Ambrose. Paulinus relates (Vita Ambrosii, xxxii-xxxiii) that Ambrose, at some time within the last three years of his life, after the death of the Emperor Theodosius (d. 395), discovered in a garden outside the walls of Milan the body of St. Nazarius, with severed head and still stained with blood, and that he caused it to be carried to the Basilica of the Apostles. In the same garden Ambrose likewise discovered the body of St. Celsus, which he caused to be transported to the same basilica. Obviously a tradition regarding these martyrs was extant in the Christian community of Milan which led to the finding of the two bodies. A later legend, without historical foundation, places the martyrdom of these witnesses to the faith during the persecution of Nero, and describes with many details the supposed journeyings of St. Nazarius through Gaul and Italy. He is also brought into relation with the two martyrs Gervasius and Protasius. Paulinus says distinctly (l.c.) that the date on which Nazarius suffered martyrdom is unknown. The discourse eulogizing the two saints, attributed to St. Ambrose (Sermo lv, in P.L., XVII, 715 sqq.), is not genuine. St. Paulinus of Nola speaks in praise of St. Nazarius in his Poema xxvii (P.L., LXI, 658). A magnificent silver reliquary with interesting figures, dating from the fourth century, was found in the church of San Nazaro in Milan (Venturi, “Storia dell’ arte italiana”, I, Milan, 1901, fig. 445-49). The feast of the two martyrs, with that of Sts. Victor and Innocent, is on 28 July. 1914 Catholic Encyclopedia
The original author of this blog passed away in July of 2016. RIP Father Carota.