This evening the Church begins the "final countdown" to celebrating our Lord's Incarnation and Birth by using the "O Antiphons" in her Vespers/Evening Prayer. One site even suggests that these next seven days in which the Church uses the "O Antiphons" are a second part to the season of Advent.
During the whole of Advent we focus on the coming of Christ, and His coming in the past, in the present, and in the future are woven together throughout the season. In the past He came in flesh and blood, in humility and death, and in resurrection victory. In the present He comes in grace and mercy by way of His Gospel proclaimed and His Sacraments given out in the life of the Church. In the future He will come again in His glorious majesty and take us to eternity to enjoy His gift of life.
As our time of preparation begins winding down, and the time of celebration is fast approaches, we sing the “O Antiphons” during Evening Prayer (Vespers). One antiphon for each of the seven days before Christmas Eve (December 17-23) is sung or spoken before and after the Psalmody. Each antiphon consists of a title given to Christ in the Old Testament followed by a petition asking Christ to come and fulfill a promise of Scripture. Thus we join with the saints of the Old Testament in longing for our Savior to come and carry out His promised deliverance.
These “O Antiphons” have been put into verse form in the very familiar Advent hymn: “O Come, O Come, Emmanuel” (LSB 357). In fact, Lutheran Service Book nicely prints "The Great 'O' Antiphons" side-by-side with the hymn. So we can incorporate the "O Antiphons" into our daily prayers in a couple of ways: singing/speaking the Antiphon itself, and/or singing the corresponding verse from "O Come, O Come, Emmanuel."