Basilicas
The Four Pilgrimage Basilicas
This is an informal description of the great Roman churches which have Holy Doors that are opened at the beginning of the Jubilee years, either by the Pope himself or by his representative. The Holy Doors are then closed and sealed at the end of the Jubilees. Traditionally, the Holy Door of Saint Peter's Basilica is opened before the Midnight Mass on Christmas Eve, that of Saint John in the Lateran, the Cathedral of Rome, on Christmas morning, that of Saint Mary Major on New Year's Day, (which is now officially designated as the Solemnity of Mary, Mother of God), and that of Saint Paul outside the Walls on the Feast of Saint Paul's Conversion, January 25th. While the Jubilee door is open, all those Catholics who fulfill the necessary conditions (confession, communion, prayer for the intentions of the Pope, etc.) can receive a Plenary Indulgence for the act of crossing through the Holy Door, as a symbol of passage from the state of sin to that of Grace.
New Advent article on Jubilees
The Five Patriarchal Basilicas
Since the fifth century, the Church has designated five of the most ancient and important episcopal sees as Patriarchates, that is, sees which have jurisdiction over the other episcopal sees of their region. These five are Alexandria, Antioch, Constantinople, Jerusalem, and Rome, the see of the Pope. To symbolize that the Pope has jurisdiction over the entire Catholic Church, including the other Patriarchates, each of these Patriarchates is represented by a church in the city of Rome; these churches are therefore called the Patriarchal Basilicas. The five Patriarchal Basilicas of Rome are the four "pilgrimage Basilicas" mentioned above, plus the church of Saint Lawrence outside the Walls on the via Tiburtina. Until the 1960's, it was the custom that only the Pope or his chosen representative could celebrate Mass on the principal altar of these churches. This custom is still observed at Saint Peter's, although it has been allowed to lapse in the other four.
The Basilica of Saint Lawrence has a rather odd appearance to it, since it is actually two churches built up against each other, later united into a single church. Under the High Altar is the tomb of Saint Lawrence, the Roman deacon who was roasted alive during the persecution of the Emperor Decius, about 250 A.D., for refusing to surrender to the Roman state the money with which the Church would feed the poor of Rome. Later, the relics of St. Stephan, the deacon of Jerusalem who is venerated by the Church as her very first martyr, were interred next to those of Saint Lawrence. This church also contains an interesting decorated crypt chapel, which houses the tomb of Blessed Pope Pius IX. Pope Pius reigned longer than any other Pope besides Saint Peter himself, nearly 32 years, and was beatified by Pope John Paul II during the Jubilee year 2000. Those who have time should also take a walk through the Campo Verano, the beautiful cemetery next to Saint Lawrence's, which has been officially designated a national monument by the Italian state.
The Seven Churches
From the sixteenth century, the "Seven Churches" has been the customary designation for a lengthy pilgrimage walk around the city of Rome. These seven churches are the five Patriarchal Basilicas, plus the churches of Saint Sebastian on the Appian Way, and the Basilica of the Holy Cross, nicknamed "in Jerusalem", where the relics of the True Cross are preserved. The precise origin of this practice is not known, but it was one of the favorite devotions of Saint Philip Neri, founder of the Oratorians, and has been especially popular with pilgrims who come to Rome during the Jubilees.
By walking the Seven Churches, one visits the tombs of the two Apostles who founded the Church in Rome, the oldest church in the world dedicated to the Virgin Mary, the Pope's own Cathedral, the tomb of Saint Lawrence, one of Rome's most famous and important martyrs, as well as the relics of the True Cross, and the only one of Rome's many Catacombs, that of Saint Sebastian, which has always been at least partially accessible to pilgrims.