Today is the third day of the misa de aguinaldo. That means we are six days away from Christmas. Before the 1960 rubrical changes raised the dignity and rank of the Advent ferias from 17 to 23 December, many countries in the Spanish realm kept the feast of the Expectation of the Blessed Virgin on 18 December, a holdover of the feast of the Annunciation transferred from 25 March by the Tenth Council of Toledo. Images of the Expectation of the Blessed Virgin usually depict Mary gravid.
The feast eventually assumed the title of Nuestra Señora de la O, literally, Our Lady of the O. What inspired Spaniards to use O traces a rather interesting history, which is ultimately, and surprisingly, choral in provenance. In summary, for seven days beginning on the eve of the feast, that is, from 17 to 23 December, at Vespers in the cathedral of Toledo, at the inchoation of the Great Antiphons, the dignity charged with the intonation prolonged the interjection O, and the rest of the choir joined with its lengthening, signifying with “these profound suspirations of longing” the “desire and yearning of the world for the Redeemer” . For more details, read this.