December 18 was a special day in the Church's calendar to contemplate this profound mystery.
Historically the Church, specifically the local church in Spain, celebrated a fascinating feast on December 18 called “The Expectation of the Blessed Virgin Mary.” It can be traced back to the 7th century and was originally the feast of the Annunciation.
At that time special feast days during Lent were not allowed and so instead of transferring the feast of the Annunciation from March 25 to a random day during Easter, the Church decided to transfer it to the Advent season. The date of December 18 was picked and the feast remained focused on the mysteries surrounding the Annunciation for several centuries.
By the 17th century the regulations during Lent were relaxed and the Church saw it more fitting to celebrate the Annunciation in March than in December. They transferred the feast back to its original date, but in Spain they kept December 18 as a special day dedicated to the Blessed Virgin Mary. Its celebration was even given an octave, making each day before Christmas a special day in honor of the Blessed Mother.
Dom Gueranger explains how “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.”
Furthermore, Gueranger adds that a tradition arose surrounding this feast day where, “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 honor our Lady’s Maternity, and beg her blessing upon themselves.”
The celebration of this feast is no longer on the universal calendar, but it does highlight an important historical reality. It invites the faithful to envision what it was like to be the Blessed Virgin Mary, about to give birth to the Savior of the world. The feast heightens the anticipation of Christmas and makes the last few days of Advent unique opportunities to meditate on what Mary must have been pondering in her heart.