The feast of the Expectation of the Blessed Virgin Mary owes its origin to the 10th Council of Toledo, in 656.
Because of the perceived incongruity of celebrating the joyful Feast of the Annunciation in the midst of Lent, the bishops of Spain moved this great feast to the 18th of December, celebrating both the Annunciation and the Divine Motherhood of the Blessed Virgin Mary on that day.
Later on, the Spanish liturgical calendar would reunite with the Roman Rite and the Annunciation would once again be celebrated on March 25. However, the devotion of the faithful was so great that the Church instituted a new feast under the title of the Expectation of the Birth of the Blessed Virgin on December 18, eight days before Christmas, in memory of the Annunciation and also as a preparation for the Nativity.
It is also called the Feast of Our Lady of the O, or the Feast of the O, because of the great Antiphons that the Church sings every day from December 17 until the day before Christmas (according to Dom Prosper Guéranger's Liturgical Institutions).
Expectation means waiting. In fact, there are two events in waiting. The Immaculate awaits her God, her divine Child. She awaits Him in peace and joy. Truly she is blessed in her expectation.
More than all the prophets, she desires this hour of deliverance and more than the Kings of the Old and New Testament, she wants to give her Son, her Saviour, to the desolate world. She wishes with all her heart for the hour when He, the Lord and Master of all, will open His gates: “Lift up your heads, O gates, stand up. Lift up your pediments, O gates, rise up, you eternal gates, that the King of glory may enter!” (Ps. 23).
Our Lord Jesus Christ is also waiting on the other side of the door. He is waiting for His hour. But throughout this waiting, what is the prayer that continually escape from the Immaculate Heart?
They are those that the Church of the first centuries has transmitted to us in this time of waiting: “O Wisdom, O Adonai, O Branch of Jesse, O Key of David, O East, O King of the Universe, and O Emmanuel, come!”
This “O” is used for invocation, introduction to prayer and supplication. It also marks astonishment, admiration, and pain.
O Immaculate One, give us your desire to see the features of the One who holds the Keys of our Salvation, Clavis David (Key of David).