Dom Prosper Gueranger:
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.[The Liturgical Year]
This feast, which sometimes goes under the name of Our Lady of O, or the feast of O, on account of the [antiphon] which begins O Virgo virginum [and because on that day the clerics in the choir after Vespers used to utter a loud and protracted O, to express the longing of the universe for the coming of the Redeemer], 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...
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.
O Virgo virginum, quomodo fiet istud? quia noc primam similem visa es, nec habere sequentem. Filae Jerusalem, quid me admiramini? Divinum est mysterium hoc quod cernitis.
O Virgin of virgins! how shall this be? for never was there one like thee, nor will there ever be. Ye daughters of Jerusalem, why look ye wondering at me? What ye behold, is a divine mystery.