The familiar expression, “When in Rome, do as the Romans do” tends to be used to express a kind of laid back attitude towards life. The origin of the expressoin is actually advice given by St. Ambrose to St. Augustine. It is often “quoted” various ways, and one would think it is easier to identify the source (I should do a search of Christian Classics Ethereal Library to see what I can find out).
In either case, the expression derives from the fact that Milan has different liturgical traditions than Rome. So, St. Ambrose said to St. Augustine, regarding the different liturgical traditions of local churches at the time, “When I am in Rome, I observe the Roman fast. When I am in Milan, I do not.”
All of that gets to the fact that there is an ancient Rite in the Diocese of Milan and certain neighboring diocese known as the Ambrosian Rite (for St. Ambrose). Apparently, just as the Dominicans and the Old Observance Carmelites adopted the Liturgy of Vatican II, so, apparently, did the Diocese of Milan.
(The Dominican Rite was almost identical to the Gregorian Rite, except for mostly technicalities about candles and stuff; the Carmelite Rite, since the Carmelites were originally a Byzantine Order, was a kind of admixture of the Byzantine and Gregorian Liturgies). Note I used the term “Gregorian” because both Orders predated the Council of Trent. When Trent revised the Gregorian Mass, and St. Teresa founded the Discalced Carmelites as part of the Counter-Reformation, she adopted the Tridentine Mass.
I’m not sure whether there’s a “Vatican II Ambrosian Mass,” or if they just use the Ordinary Form of the Roman Rite. However, apparently the bishops of Milan have been even more hostile to retaining their unique liturgical tradition than bishops in the Roman Rite (this also gets to the difference between Rite and Church, since Rome and Milan have different Rites but are subject to the same Patriarch and the same Code of Canon Law).
So, the Pontifical Commission Ecclesia Dei has ruled that the principles of Summorum Pontificum apply to those who are members of the Diocese of Milan (and relevant adjacent diocese) who wish to practice *their* traditional liturgy, as well as to those who want the traditional Roman Missal.
Here’s what I wonder: if Summorum Pontificum and Cardinal Ratzinger’s pre-papal writings, along with St. Pius V’s Quo Primum, contend that no ancient Rite can be “abrogated,” what are the rules regarding a priest *outside* the Diocese of Milan?
Do Catholics interested in liturgical diversity and heritage have to fly to Milan, or fly a priest and schola *from* Milan, in order to experience the Ambrosian Rite? Can a priest, by the principles of Summorum Pontificum, study the Ambrosian Rite and practice it *outside* those dioceses?
I think it would be cool to start a “Shrine of the Universal Church,” where every liturgical tradition is represented at least once per year, if not regularly, at Mass, with the bare minimum being a Latin Paul VI Mass, a mass in the extraordinary Roman form, and a Byzantine Divine Liturgy.