"The Blessed Virgin Mary Looking Shortly to be Delivered"
From Fr Mark's Vultus Christi:
Yes, today, December 18th, is one of the liturgy's loveliest old Advent festivals of the Blessed Virgin Mary, that of the Expectatio Partus. Established in 656 by the bishops assembled for the Tenth Council of Toledo, it was kept by nearly the entire Latin Church. Mother Mectilde de Bar, writing in 17th century France, left some splendid sermons on the feast. The Marquess of Bute calls it, in his fine old translation of the Breviary, "The Blessed Virgin Mary Looking Shortly To Be Delivered." It was also called in Spain, and elsewhere, Nuestra Señora de la O, and this because, after Vespers, the clergy in choir used to give voice to a loud and protracted "O" to express the yearning of the universe for the advent of the Redeemer.
Do click and read the rest of it here.