As stated by the great Liturgist Dom Gueranger:
On the twenty-ninth of June, in the year 67, whilst Peter, having crossed the Tiber by the triumphal bridge, was drawing nigh to the cross prepared for him on the Vatican plain, another martyrdom was being consummated on the left bank of the same river. Paul, as he was led along the Ostian Way, was also followed by a group of the faithful who mingled with the escort of the condemned.
His sentence was that he should be beheaded at the Salvian waters. A two miles' march brought the soldiers to a path leading eastwards, by which they led their prisoner to the place fixed upon for his martyrdom. Paul fell on his knees, addressing his last prayer to God; then having bandaged his eyes, he awaited the death-stroke. A soldier brandished his sword, and the apostle's head, as it was severed from the trunk, made three bounds along the ground; three fountains immediately sprang up on these several spots.
Such is the local tradition; and to this day three fountains are to be seen on the site of his martyrdom, over each of which an altar is raised.COLLECT PRAYER
O God, You have instructed many nations through the preaching of the blessed apostle Paul. Let the power of his intercession with You help us who venerate his memory this day. Through our Lord...
COMMEMORATION OF SAINT PETER
O God, You have entrusted the keys of the kingdom of heaven to Your blessed apostle Peter and have given him the power of bishop to bind or to loose. May his intercession free us from the slavery of sin; who lives and rules with God the Father . . .