Of 17 December
As a liturgical season, Advent probably developed outside of Rome. By the fourth century, the Council of Saragossa urged the constant presence of the faithful in the church for twenty-one continuous days from December 17 to January 6, the Feast of the Epiphany. In spite of the many changes that have happened in the Church since the fourth century, December 17 is still the beginning of the "Great O" antiphons that characterize our liturgical prayer during the days leading up the Solemnity of the Nativity of Our Lord Jesus Christ.
Most of us know these antiphons as they are used as the verses for a very well-known and recognized Advent chant: "O Come, O Come, Emmanuel." Today on 17 December we pray: "O Wisdom of our God Most High, guiding creation with power and love: come to teach us the path of knowledge!"
Wisdom offers us a self-description in the Book of Sirach:
"From the mouth of the Most High I came forth,
and mistlike covered the earth.
Before all ages, in the beginning, God created me,
and through all ages I shall not cease to be.
Come to me, all you that yearn for me,
and be filled with my fruits." (Sirach 24: 3, 9, 18)