St. Ambrose, Doctor of the Church, and Bishop of Milan. The teacher of St. Augustine, and steadfast opponent of the Arian heresy. Noted rhetorician, writer, and preacher. St. Augustine was converted by hearing his sermons and baptized by him.
In the Litany of the Saints, he follows St. Gregory the Great, and precedes St. Augustine, and so is second of the first four doctors of the Church listed in the litany, the fourth being St. Jerome. This group of four saints is ranked above St. Martin and St. Nicholas, whose feast was yesterday -- not that the Litany is a precise ranking of saints, but merely a useful order for considering them.
Ora pro nobis, Sancte Ambrosi.