The Saturday in Ember Week in Advent -- which is today in the calendar of the usus antiquior -- gets a significant treatment in Blessed Cardinal Schuster's work, The Sacramentary. This is for reason of the particular length of the Mass of today, and also because of the historical association of this day with ordinations in Rome.
At first the ordinations of the sacred ministers in Rome took place only in the month of December -- when, that is to say, the Christian family at the approach of the Christmas festival made an offering, as it were, to God, by a solemn three day's fast... taking this opportunity to beg the bestowal of his gifts upon those whom the Holy Ghost had chosen to carry on the work of the Apostles to guide the flock of Jesus Christ...
-- p. 337
To this the Catholic Encyclopedia's entry on "Holy Orders" adds: "During the first centuries ordination took place whenever demanded by the needs of the Church. The Roman pontiffs generally ordained in December (Amalarius, "De offic.", II, i). Pope Gelasius (494) decreed that the ordination of priests and deacons should be held at fixed times and days, viz., on the fasts of the fourth, seventh, and tenth months, also on the fasts of the beginning and midweek (Passion Sunday) of Lent and on (holy) Saturday about sunset (Epist. ad ep. Luc., xi). This but confirmed what Leo the Great laid down, for he seems to speak of ordination on Ember Saturdays as an Apostolic tradition (Serm. 2, de jejun. Pentec.)."
Cardinal Schuster continues on with a more general historical treatment of Ember Saturdays:
In the old sacramentaries the Ember Saturdays are often called the Saturdays of the Twelve Lessons... Long before the monks transplanted from Egypt the form of the psalmodic vigil in use in those monasteries and introduced it into the liturgy of the Roman basilicas, the night watch of the Church in Rome adopted a complete intermingling of twelve lessons, repeated in Latin and Greek... and alternated with the responsorial chant of the famous morning Hymns and with the collects of the priest... At the end of the vigils, at daybreak, the canticle of the three youths of Bablyon, commonly called the Benedictiones, brought the psalmody to a close, and served to fill in the time between the vigiliary office and the offering of the eucharistic sacrifice. Before, however, the sacred gifts were brought to the altar, the ordination of the new ministers took place...
We know as a matter of fact that it was St. Gregory who shortened the primitive vigiliary rite, which originally included the recitation of twelve lessons in Greek as well as in Latin. The holy Pontiff reduced these by half, but such was the force of habit that outside the immediate surroudings of the papal Curia not only did the old name of Saturday of the Twelve Lessons, already given to the Ember Saturdays, remain unchanged, but, thanks to the Gelasian Sacramentary, which was in use in very many places in France and elsewhere, the famous twelve lessons of the Easter vigil also escaped destruction.
-- p. 337-338
If we look at the Mass for today in the liturgical books of the usus antiquior, "Sabbato Quatuor Temporum Adventus", one will see that following the Introit and Kyrie we have the "Flectamus genua" just as we did on Ember Wednesday -- which, as Schuster notes, "is a last relic of the procession to the stational church which formerly took place."
Following these we see the reduced number of lessons mentioned above. The sequence is as follows:First Lesson: Isaiah 19: 20-22
Second Lesson: Isaiah 35: 1-7
Third Lesson: Isaiah 40: 9-11
Fourth Lesson: Isaiah 45: 1-8
Fifth Lesson: Daniel 3: 47-51
Each of these lessons, except the fifth, is followed by a gradual taken from the psalms, by a call to again kneel (Flectuamus genua) and a prayer.
The fifth lesson from the Book of the Prophet Daniel proceeds straight into the Canticle of the Three Youths, taken from Daniel 3: 52-56. This is followed by a prayer, but without the flectamus genua.
The final place in the sacred vigils is always reserved for Daniel and for the moving scene of the three youths cast into the furnace of Babylon, which serves as a prelude to the grand canticle of the Benedictiones, freely adapted from the great Hallel of the Psalter.
-- Ibid. p. 348
Following this, an Epistle is read, taken from St. Paul's Second Letter to the Thessalonians, 2: 1-8.
This is followed finally by the Gospel.
