In 1579, the Spanish governor of Milan, Italy, Governor Ayamontem, ordered the carnival to continue through the first Sunday of Lent. The calendar of the Roman Rite that was used throughout the Church suggested that self-denial begins on Ash Wednesday, before the first Sunday of Lent. But the governor was right in that Milan didn’t follow the Roman Rite. Instead it used the Ambrosian Rite. According to the Ambrosian Rite, fasting and abstinence begins on the Monday following the first Sunday of Lent; the Sunday Mass is festive with vestments of white and a chanting of the Gloria and Alleluia. Carnival on the first Sunday of Lent would be consistent with the festive atmosphere then under the Ambrosian Rite.
Yet the Council of Trent asked for uniformity throughout the Catholic Church. Some asked whether the Church would break up into little segments, each having different rules and beliefs as the Protestants were beginning to do. Still, the council gave an exception to that rule of uniformity. It said that local rites that could show usage over two centuries might be retained. The Ambrosian Rite is named after St. Ambrose, who added hymns to the rite in the fourth century. It has always been used in the Archdiocese of Milan. In fact, some suggest that the Ambrosian Rite isn’t an ancient Roman liturgy at all, that the original was imported from Ephesus in Asia Minor in the second century by St. Irenaeus, who had received it through St. Polycarp from St. John the Apostle.
St. Charles Borromeo, as Archbishop of Milan, didn’t want to destroy the rite’s Ambrosian characteristics. After all, he was Milanese. He agreed that Mass wouldn’t be said during the Fridays of Lent, nor communion distributed. All that would stay the same. However, he proposed that when the rest of the Catholic Church was complying with the Council of Trent and fasting on the first Sunday of Lent, Melanese should not participate in a carnival. Governor Ayamonte disagreed for the income to Milan was substantial from carnival. Most of Milan supported the governor. The king of Spain said the pope should decide.
Pope Gregory supported Archbishop Borromeo and carnival ended on Ash Wednesday. However, other aspects of the Ambrosian Rite together with the Ambrosian Breviary continued to be used in spite of a new missal and breviary used in the universal Church after the Council of Trent. See The Hidden Saint: The 16th Century Church in Crisis for more like this.