Explanation adapted from The Roman Catholic Lectionary Website
compiled by Felix Just, S.J., Ph.D.
For the Week before Christmas, from December 17 to 23, there are special Masses and Lectionary Readings that take precedence over the ordinary weekdays (but not Sundays) of Advent.
For these seven days, during the Evening Prayer of the Liturgy of the Hours (Vespers) the Antiphons that introduce the reciting or singing of Mary’s hymn, the Magnificat (Luke 1:46-55), all begin by addressing Jesus by a special title preceded by the exclamation “O.” They are called the “O Antiphons.” Each Antiphon calls on the Messiah to come, beginning with a biblical title and closing with a specific petition.
These seven traditional "O Antiphons" are more than a thousand years old. Since the Second Vatican Council, they have been adapted (slightly reworded and rearranged) for the "Alleluia Verse" of the Mass:
O Wisdom (Sapientia) of our God Most High, guiding creation with power and love: come to teach us the path of knowledge!
O Leader (Adonai) of the House of Israel, giver of the Law to Moses on Sinai: come to rescue us with your mighty power!
O Root (Radix) of Jesse’s stem, sign of God's love for all his people: come to save us without delay!
O Key (Clavis) of David, opening the gates of God's eternal Kingdom: come and free the prisoners of darkness!
O Radiant Dawn (Oriens), splendor of eternal light, sun of justice: come and shine on those who dwell in darkness and in the shadow of death!
O King (Rex) of all nations and keystone of the Church: come and save us, whom you formed from the dust!
O Emmanuel (same), our King, and Giver of Law: come to save us, Lord our God!
In the reflections that follow, the Antiphons are a more literal translation of the Latin.
In the traditional arrangement, when viewed from Christmas Eve backward, the first letters of the Latin texts (Emmanuel, Rex, Oriens, Clavis, Radix, Adonai, Sapientia) spell out the phrase ero cras ("I will be here tomorrow").