Today in the pre-1955 Traditional Catholic Missal is the Mass in Some Places (pro aliquibus locis) of The Expectation of the Blessed Virgin Mary. The history of this special feast is related by the great liturgist of the modern era, Dom Prosper Guéranger in his Liturgical Year.
THIS feast, which is now kept not only throughout the whole of Spain but in many other parts of the Catholic world, owes its origin to the bishops of the tenth Council of Toledo, in 656. These prelates thought that there was an incongruity in the ancient practice of celebrating the feast of the Annunciation on the twenty-fifth of March, inasmuch as this joyful solemnity frequently occurs at the time when the Church is intent upon the Passion of our Lord, so that it is sometimes obliged to be transferred into Easter time, with which it is out of harmony for another reason; they therefore decreed that, henceforth, in the Church of Spain there should be kept, eight days before Christmas, a solemn feast with an octave, in honour of the Annunciation, and as a preparation for the great solemnity of our Lord’s Nativity.. In course of time, however, the Church of Spain saw the necessity of returning to the practice of the Church of Rome, and of those of the whole world, which solemnize the twenty-fifth of March as the day of our Lady’s Annunciation and the Incarnation of the Son of God. But such had been, for ages, the devotion of the people for the feast of the eighteenth of December, that it was considered requisite to maintain some vestige of it. They discontinued, therefore, to celebrate the Annunciation on this day; but the faithful were requested to consider, with devotion, what must have been the sentiments of the holy Mother of God during the days immediately preceding her giving Him birth. A new feast was instituted, under the name of ‘the Expectation of the blessed Virgin’s delivery.’
This feast, which sometimes goes under the name of Our Lady of O, or the feast of O, on account of the great antiphons which are sung during these days, and, in a special manner, of that which begins O Virgo virginum (which is still used in the Vespers of the Expectation, together with the O Adonaï, the antiphon of the Advent Office), is kept with great devotion in Spain. A High Mass is sung at a very early hour each morning during the octave, at which all who are with child, whether rich or poor, consider it a duty to assist, that they may thus honour our Lady’s Maternity, and beg her blessing upon themselves. It is not to be wondered at that the holy See has approved of this pious practice being introduced into almost every other country. We find that the Church of Milan, long before Rome conceded this feast to the various dioceses of Christendom, celebrated the Office of our Lady’s Annunciation on the sixth and last Sunday of Advent, and called the whole week following the Hebdomada de Exceptato (for thus the popular expression had corrupted the word Expectato). But these details belong strictly to the archæology of liturgy, and enter not into the plan of our present work ; let us, then, return to the feast of our Lady’s Expectation, which the Church has established and sanctioned as a new means of exciting the attention of the faithful during these last days of Advent.
Most just indeed it is, O holy Mother of God, that we should unite in that ardent desire thou hadst to see Him, who had been concealed for nine months in thy chaste womb ; to know the features of this Son of the heavenly Father, who is also thine; to come to that blissful hour of His birth, which will give glory to God in the highest, and, on earth, peace to men of good-will. Yes, dear Mother, the time is fast approaching, though not fast enough to satisfy thy desires and ours. Make us redouble our attention to the great mystery; complete our preparation by thy powerful prayers for us, that when the solemn hour has come, our Jesus may find no obstacle to His entrance into our hearts.
Gregory DiPippo adds from a 2020 Facebook Page:
Remember to pray the O Antiphon for December 18th today during Vespers along with the followingThe feast of Our Lady's Expectation (Expectatio Partus) is a Spanish feast, which derives ultimately from the Mozarabic custom of keeping the feast of the Annunciation on Dec. 18. Even though the Mozarabic liturgy had almost completely disappeared by the early 15th century, this particular feast was evidently popular enough that they decided to insert it into the local Uses of the Roman Rite. At Badajoz, Burgos and Segovia, it was just called "the Annunciation", in Avila, Braga, Seville, and Compostela "the Commemoration of the Annunciation", at Palencia, either just "the Annunciation" or "the repeated solemnity of the Annunciation", and at Valladolid, "the festivity of St Mary before Christmas."
In many pre-Tridentine Spanish Missals, the Mass on Dec. 18 was therefore just a repetition of the Mass of March 25, sometimes with a different Collect, sometimes also with a different Secret and/or post-Communion, and often with a Sequence. In some other places, however, the Mass for the Expectation was the same one included in the supplement "pro aliquibus locis" in many editions of the Roman Missal, which is Roman Mass of Ember Wednesday "Rorate caeli", remodeled as a festive Mass by using the prayers of March 25th and adding an Alleluja.
O Virgo Virginum: