The O Antiphons: Introduction

From Wikipedia:

The O Antiphons are antiphons used at daily prayer in the evenings of the last days of Advent in various liturgical Christian traditions.

Each antiphon is a name of Christ, one of his attributes mentioned in Scripture. They are:

December 17: O Sapientia (O Wisdom)
December 18: O Adonai (O Adonai)
December 19: O Radix Jesse (O Root of Jesse)
December 20: O Clavis David (O Key of David)
December 21: O Oriens (O Morning Star)
December 22: O Rex Gentium (O King of the nations)
December 23: O Emmanuel (O Emmanuel)

In the Roman Catholic tradition, the O Antiphons are sung or recited at Vespers from December 17 to December 23 inclusive...

The hymn O come, O come, Emmanuel (in Latin, Veni Emmanuel) is a lyrical paraphrase of these antiphons in reverse order.

The first letters of the titles taken backwards form a Latin acrostic of "Ero Cras" which translates to "Tomorrow, I will come", mirroring the theme of the antiphons.

According to Fr. William Saunders:

The exact origin of the "O Antiphons" is not known. Boethius (480–524/5) made a slight reference to them, thereby suggesting their presence at that time. At the Benedictine Saint Benedict Abbey of Fleury (now Saint-Benoît-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 were 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 Western liturgical tradition since the very early Church.
The Benedictine monks arranged these antiphons with a definite purpose. If one starts with the last title and takes the first letter of each one—Emmanuel, Rex, Oriens, Clavis, Radix, Adonai, Sapientia—the Latin words ero cras are formed, meaning, "Tomorrow, I will come". Therefore Jesus, whose coming Christians have prepared for in Advent and whom they have addressed in these seven Messianic titles, now speaks to them: "Tomorrow, I will come." So the "O Antiphons" not only bring intensity to their Advent preparation, but bring it to a joyful conclusion.

...Although the antiphons and dates shown above have been fairly universally recognised throughout western Christendom, an alternative English medieval practice arose of moving all of the antiphons forward by one day (commencing therefore on 16 December) and adding an additional (eighth) antiphon on 23 December, with the acrostic thus becoming Vero cras, "truly, tomorrow". This is the antiphon O Virgo virginum (O Virgin of virgins).

So, from the 17th to the 24th, I'll try to post the respective antiphons for the day.