December 17th marks the beginning of the Octave prior to Christmas and thus the ancient and beautiful "O antiphons" sung before the Magnificat during vespers (evening prayer). Sung each evening until Christmas Eve, the antiphons refer to the prophecy of Isaiah of the coming of the Messiah, as well as highlight the titles of Christ.
• 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)
Working backwards, the first letter of each title forms the Latin acrostic Ero Cras, "Tomorrow I will come". We echo Isaiah's prophecy and the Messiah answers.
O Sapientia: “O Wisdom, O holy Word of God, you govern all creation with your strong yet tender care. Come and show your people the way to salvation.” Isaiah had prophesied, “The spirit of the Lord shall rest upon him: a spirit of wisdom and of understanding, a spirit of counsel and of strength, a spirit of knowledge and fear of the Lord, and his delight shall be the fear of the Lord.” (11:2-3), and “Wonderful is His counsel and great is His wisdom.” (28:29).From the Dominican Antiphonarium: