This Feast is celebrated every year on November 9, and every few years it falls on a Sunday.
Via the Catholic Encyclopedia:
This is the oldest, and ranks first among the four great “patriarchal” basilicas of Rome. The site was, in ancient times, occupied by the palace of the family of the Laterani. A member of this family, P. Sextius Lateranus, was the first plebian to attain the rank of consul.
In the time of Nero, another member of the family, Plautius Lateranus, at the time consul designatus was accused of conspiracy against the emperor, and his goods were confiscated. Juvenal mentions the palace, and speaks of it as being of some magnificence, “regiæ ædes Lateranorum”. Some few remains of the original buildings may still be traced in the city walls outside the Gate of St. John, and a large hall decorated with paintings was uncovered in the eighteenth century within the basilica itself, behind the Lancellotti Chapel.
A few traces of older buildings also came to light during the excavations made in 1880, when the work of extending the apse was in progress, but nothing was then discovered of real value or importance. The palace came eventually into the hands of Constantine, the first Christian emperor, through his wife Fausta, and it is from her that it derived the name by which it was then sometimes called, “Domus Faustæ”.
Constantine must have given it to the Church in the time of Miltiades, not later than about 311, for we find a council against the Donatists meeting within its walls as early as 313. From that time onwards it was always the centre of Christian life within the city; the residence of the popes and the cathedral of Rome. The latter distinction it still holds, though it has long lost the former. Hence the proud title which may be read upon its walls, that it is “Omnium urbis et orbis ecclesiarum mater, et caput”. …
It has no saint buried beneath it, since it was not, as were almost all the other great churches of Rome, erected over the tomb of a martyr. It stands alone among all the altars of the Catholic world in being 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. It was carefully preserved through all the years of persecution, and was brought by Constantine and Sylvester from St. Pudentiana’s, where it had been kept till then, to become the principal altar of the cathedral church of Rome.
It is now, of course, enclosed in a larger altar of stone and cased with marble, but the original wood can still be seen. A small portion was left at St. Pudentiana’s in memory of its long connection with that church, and is still preserved there.
Above the High Altar is the canopy or baldacchino already mentioned, a Gothic structure resting on four marble columns, and decorated with paintings by Barna of Siena. In the upper part of the baldacchino are preserved the heads of the Apostles Peter and Paul, the great treasure of the basilica, which until this shrine was prepared to receive them had always been kept in the “Sancta Sanctorum”, the private chapel of the Lateran Palace adjoining.
Behind the apse there formerly extended the “Leonine” portico; it is not known which pontiff gave it this name. At the entrance there was an inscription commemorating the dream of Innocent III, when he saw the church of the Lateran upheld by St. Francis of Assisi. On the opposite wall was hung the tabula magna, or catalogue of all the relics of the basilica, and also of the different chapels and the indulgences attached to them respectively. It is now in the archives of the basilica.