The Duomo was dedicated to the Nativity of the Virgin Mary by San Carlo Borromeo. The Church celebrates the occasion on 8 September and the consecration ceremony, strongly desired by the Archbishop, took place on 20 October 1577.
The Archives of the Veneranda Fabbrica, the records and memory of his work, contains a 17th century copy of the account of that day in which Carlo Borromeo's devotion to the Virgin Mary is explicitly narrated. In the customary opening formulas alongside the date are the names of Pope Gregory XIII and King Philip II of Spain who was also Duke of Milan.
The date chosen by San Carlo for the celebration is in line with the most ancient tradition of the Ambrosian Church, which has always linked the day celebrating the Cathedral to the third Sunday of October, a date connected with the time when the two basilicas of Santa Maria Maggiore and Santa Tecla were still standing. On that day, in fact, the bishop's throne was carried from the summer church (Santa Tecla) to the winter Cathedral (Santa Maria Maggiore), in memory of the dedication of both basilicas. The date was so deeply rooted in the Ambrosian celebrations that, when Pope Martin V, on his return journey from the Council of Constance in 1418, was invited to consecrate the high altar of the Duomo, it was chosen as the day of the ceremony, which in that year fell on 16 October.
The dedication to the Birth of the Virgin Mary is visible in the Latin inscription "Mariae nascenti" on the façade, above the large bas relief decorating the main door of the Cathedral. Composed of loose papers, registers and parchments, the Archives today offer the citizens of Milan and other visitors formidable snapshots of the past to reconstruct, document after document, the great story of the Fabbrica.