Prior to the liturgical revisions of the late 1960s, editions of the Missale Romanum regularly included an interesting appendix-like section generally referred to as Missae pro aliquibus locis (MPAL) - officially Missae propriae quae in aliquibus locis celebrari possunt (“Proper Masses which may be celebrated in various places”). Among the many Masses conceded to various places was one for today, December 18, In Expectatione Partus Beatae Mariae Virginis (in English commonly called the “Expectation of Mary” or “Our Lady in Expectation”).
Once celebrated widely in the Western Church, the feast of the Expectation of Mary was a duplicate of the Annunciation, (March 25) owing originally to a certain squeamishness about celebrating feasts during Lent. Long after that concern had been overcome and all sorts of feasts had come to be celebrated in Lent, the duplicate Annunciation celebration on December 18 remained popular in many places. In Spain, it was known as Santa Maria de la O – apparently because on that day after Vespers the clergy in the choir used to sing a loud and protracted "O", to express the universe’s longing for its Redeemer.
The author of the famous 15-volume commentary The Liturgical Year, the 19th century liturgist Dom Prosper Gueranger (1805-1875), who re-founded Benedictine monastic life in France after the Revolution, ably expressed the sentiment at the heart of the December 18 feast and indeed of Advent as a whole:
“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.”