Ecclesiastical terminology can sometimes be confusing. For instance, what is the difference between a cathedral and a basilica. Today's liturgical feast marks the dedication of a cathedral in Rome, St. John Lateran.
The word "cathedral" comes from the Latin "cathedra," which, in English, is translated "chair." Every bishop has a chair or seat. The building or church in which that chair is located is a cathedral. Here in Chicago, the archbishop has his chair in Holy Name Cathedral. Each diocese and archdiocese in the church has a church that is designated as its cathedral. In the Archdiocese of Rome, that church is St. John, located on the Lateran Hill. Interestingly enough, although it is known as St. John, its proper name is Christ the Savior Cathedral.
The word "basilica" comes from the Greek and means a royal chamber. In modern usage it designates churches of a particular character and rank. They are usually shrine churches and the objects of pilgrimage. "Papal" basilicas are reserved to the Pope. In other words, only the Pope may celebrate Mass on the main altar of a papal basilica. There are only five such basilicas. Minor basilicas are churches which have been honored by the Pope as places of special import in the lives of the people. For instance, here in Chicago we have the Basilica of Our Lady of Sorrows, so honored because many people have made pilgrimage to this church. Devotion to Our Lady of Sorrows is particularly strong during times of war when mothers pray for the safety of their children in the armed forces.
St. John Lateran is both a cathedral and a basilica. The Church celebrates the anniversary of the dedication of this cathedral since it is regarded as the Mother Church of the entire Catholic Faith. Because the Church bears the title of Christ our Savior, today's feast is considered a Feast of the Lord. When November 9 falls on a Sunday, the Feast of the Dedication of St. John Lateran takes precedence over the Sunday liturgy.
Most major cities with large Catholic populations have at least one basilica. Each diocesan see has at least one cathedral and sometimes a second, known as a pro-cathedral. This is especially true in dioceses which cover large territories making it difficult for people to come to the cathedral.
If you are wondering why Christ the Savior Cathedral is popularly known as St. John Lateran, that designation comes from the fact that the names of St. John the Evangelist and St. John the Baptist are chiseled into the stone of the façade of the cathedral.