On 17 December we enter the Octave before Christmas and we use, at Vespers, the seven "O Antiphons". Each day until 24 December has a different one of the seven O Antiphons preceding the Magnificat.
Fr William Saunders at Catholic Education says
The exact origin of the O Antiphons is not known. Boethius (c. 480-524) made a slight reference to them, thereby suggesting their presence at that time. At the Benedictine abbey of Fleury (now Saint-Benoit-sur-Loire), these antiphons were recited by the abbot and other abbey leaders in descending rank, and then a gift was given to each member of the community. By the eighth century, they are in use in the liturgical celebrations in Rome. The usage of the “O Antiphons” was so prevalent in monasteries that the phrases, “Keep your O” and “The Great O Antiphons” were common parlance. One may thereby conclude that in some fashion the “O Antiphons” have been part of our liturgical tradition since the very early Church.
The "O Antiphons" are important for two reasons. Firstly, each antiphon highlights a title for the Messiah, and secondly each one refers to the prophecy of Isaiah telling of the coming of the Messiah.
I hope to post the O Antiphon of the day, in Latin and English, each day from 17 to 24 December. The Latin will be as per the Breviarium Romanum and the English as per the The Divine Office translation as approved for use in England (and most other English speaking countries bar the US). Quotations from scripture will be as appear in The Jerusalem Bible.