St. John Lateran

Source: The Liturgy of the Hours – Office of Readings
The anniversary of the dedication of the Lateran Basilica, which was erected by the Emperor Constantine in 333, has been observed on this day since the twelfth century. This is the oldest, and ranks first among the four great “patriarchal” basilicas of Rome. The basilica was then, and remains today, the official cathedral of the Bishop of Rome, the Pope.
Saint Leo the Great restored it around the year 460. This second church lasted for four hundred years and was then burnt down. It was rebuilt by Clement V and John XXII, only to be burnt down once more in 1360, but again rebuilt by Urban V.
In 1376, when the Gregory XI and the papal court returned to Rome after residing for 74 years in Avignon, France, they found the city deserted and the churches almost in ruins. Great works were begun at the Lateran by Martin V and his successors. The palace, however, was never again used by them as a residence. The Vatican, which stands in a drier and much higher location, was chosen in its place. It was not until the latter part of the seventeenth century that the church took its present appearance. It’s altar stands alone among all the altars of the Catholic world in being built of wood and not of stone, and enclosing no relics of any kind. The reason for this peculiarity is that it is itself a relic of a most interesting kind, being the actual wooden altar upon which St. Peter is believed to have celebrated Mass during his residence in Rome.
This feast was at first observed only in Rome but later in honor of the basilica, which is called the mother church of Christendom, the celebration was extended to the whole Latin Church. This action was taken as a sign of devotion to and of unity with the Chair of Peter which, as Saint Ignatius of Antioch wrote, “presides over the whole assembly of Christians.” Saint John Lateran remains the official basilica of which all popes are identified with in their title “Bishop of Rome.”