Sant'Ambrogio is one of the oldest churches in Milan and still a parish. Above the crypt stands the golden altar made by the goldsmith Vuolvinio in 835, with scenes from the life of Christ on the front and from the life of Ambrose on the back. The small chapel of San Vittore in Ciel d'Oro, once a separate building, has a mosaic that the city's tourism office calls the oldest known depiction of the bishop. Milan keeps its own Ambrosian rite, so Mass here differs in some details from the Roman rite most pilgrims know. Ambrose's feast is December 7.
Good to know before you go: the basilica closes at midday. It lists its opening as morning until midday and again from early afternoon until evening Monday to Saturday, and similar hours, a little later, on Sundays and holidays. Sightseeing is asked to keep to the late morning and early afternoon hours Monday to Saturday and the afternoon on Sunday, and never during a service. Mass is celebrated in the morning and the early evening Monday to Saturday, and on Sundays several times through the morning, at midday in Latin, and again in the evening, with sung Vespers in the late afternoon. Summer brings a reduced schedule. The nearest metro stop is Sant'Ambrogio on line M2.
The same day in Milan can take in the baptistery beneath the cathedral, identified with Ambrose's baptism of St. Augustine. Augustine's own tomb is in Pavia, a short rail journey south.