Advent is a special Marian liturgical season, as it focuses on Mary's role in salvation history, culminating in her giving birth to the Savior of the world.
For many centuries the Church celebrated a special Marian feast on December 18 called the Expectation of the Blessed Virgin Mary.
As Dom Prosper Guéranger explains, "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.”
He goes on to say, "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."
The following O Antiphon was recited or sung on this day:
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.
The feast has been removed from the universal calendar of the Church, but can still help our final preparations for Christmas.
It reminds us of those final days and invites us to imagine what it would have been like for Mary, patiently waiting to give birth to the Savior.