Well, actually, on the Church's calendars--both new and traditional-- today's feast is that of the great St. Ambrose, one of the Fathers of the Western Church. However, before the calendar changed in 1955, the Vigil of the Immaculate Conception was observed liturgically today.
Wikipedia briefly discusses the 1955 rubrical and calendar changes:
The rubrics and calendar of the Mass and the Divine Office were reformed by the constitution Cum hac nostra aetate (March 23, 1955). The reform to the calendar, the most dramatic before its complete overhaul in 1969, consisted mainly in the abolition of various octaves and vigils. An octave is the week-long prolongation of a great feast, either by the celebration of a proper Mass all through the Octave or by the addition of an additional Collect when the Mass of another feast is celebrated. Of the 18 octaves existing in the Roman calendar, all but three (Easter, Pentecost, Christmas) were purged in the reform, including the octaves of the Epiphany, Corpus Christi, the Ascension and the Immaculate Conception. A vigil is a day of liturgical preparation preceding a great feast. The reform of 1955 eliminated roughly half the vigils in the Roman calendar, including the vigil of the Epiphany and the vigils of the Apostles.