To add to the solemnity of the feast of the Immaculate Conception of Our Blessed Lady, Pope Leo XIII in 1879 extended this vigil to the Universal Church.
(Source: The New Roman Missal, Rev. F. X. Lasance)
St. Ambrose, a doctor or teacher of the Church, was Archbishop of Milan from A.D. 374 to his death in 397. By his steadfastness he deserved well of God”s people. Gentleness, meekness, humility, and obedience mad him yield to every one in indifferent matters, but in those of duty was inflexible. His writings have contributed many hymns and lections to the Roman Breviary. St. Ambrose died April 4, 397; but the fourth of April falling as a rule in Lent, the seventh of December, anniversary of his consecration as bishop, was assigned for his festival.