The two basilicas of the apostles were held in great veneration from the earliest times. The priest Caius, declared against heretics: “I can show you the trophies of the Apostles; for if you will go to the Vatican on the Ostian Way you will find the trophies of those who founded this Church.” Acceding to the request of Pope Sylvester, Constantine undertook to make the sanctuaries at the tombs of these apostles correspond externally with their importance in the eyes of the whole Christian world. He erected the ancient Basilica of St. Peter on the Vatican and that of St. Paul outside the walls. On November 9, the Dedication of the Basilica of the Saviour at Rome; on November 18, that of the Basilicas of the Apostles is celebrated. The reason for determining November 18 is unknown. It would appear as though these dedications occurring within a few days of each other had been fixed by some conventional arrangement.
The present St. Peter”s is on the site of the ancient church, marking the spot where St. Peter was martyred and buried. It was built in the Sixteenth Century when Pope Julius II and Leo X engaged Bramante and Michael Angelo to construct what is now the most vast and the richest church in the world. The new Church was consecrated by Urban VIII on November 18, 1626.
In 1823 the Basilica of St. Paul was almost wholly destroyed by fire, which spared only that part of the Church were the altar of the Apostle stands under the triumphant arch of Leo the Great. It was rebuilt by the efforts of successive Popes and reconsecrated by Pius IX on December 10, 1854.
So by the wonderful dispensation of Providence it came to pass that the Catholic Church celebrates annually the dedication of the four patriarchal basilicas at Rome: that of the Savior, of St. Peter, St. Paul and St. Mary Major. As each diocese commemorates the encoenia of its own cathedral, so the whole Catholic world celebrates annually the dedication of the fourfold Papal Cathedral, and this festival is symbolical of the fact that in spite of the limits established to each diocese the Church of Christ is one, and is founded on Peter, who continues to feed his lambs from the seven hills, and to rule over the flock of Christ throughout the earth.
(Source: The New Roman Missal, Rev. F. X. Lasance)