December 17th begins the week-long countdown until Christmas - during the final days of Advent you find the 'O' Antiphons present in the liturgy (an antiphon is basically a repeated verse - think of the Responsorial Psalm at Mass - the verse that the congregation repeats is an antiphon). These are the antiphons that begin and end the Magnificat in Vespers during the final days of Advent and are called the 'O' Antiphons simply because they all begin with 'O', and are verses invoking titles of Christ as you can see from the Breviary:
Dec. 17th: 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.
Dec. 18th: O sacred Lord of ancient Israel, who showed yourself to Moses in the burning bush, who gave him the holy law on Sinai mountain: come, stretch out your mighty hand to set us free.
Dec. 19th: O Flower of Jesse's stem, you have been raised up as a sign for all peoples; kings stand silent in your presence; the nations bow down in worship before you. Come, let nothing keep you from coming to our aid.
Dec. 20th: O Key of David, O royal power of Israel controlling at your will the gate of heaven: come, break down the prison walls of death for those who dwell in darkness and the shadow of death; and lead your captive people into freedom.
Dec. 21st: O Radiant Dawn, splendor of eternal light, sun of justice: come, shine on those who dwell in darkness and the shadow of death.
Dec. 22nd: O King of all the nations, the only joy of every human heart; O Keystone of the mighty arch of man, come and save the creature you fashioned from the dust.
Dec. 23rd: O Emmanuel, king and lawgiver, desire of the nations, Savior of all people, come and set us free, Lord our God.
In Latin the titles are Sapientia (Wisdom), Adonai (Lord), Radix Jesse (Root of Jesse), Clavis David (Key of David), Oriens (Dawn), Rex Gentium (King of the nations).
Here is a resource with scriptural citations of each of the titles and the Latin text of the antiphons followed by a good English translation - and you can hear the Latin antiphon chanted (which is awesome!) by clicking on the speaker icon.
Fr Z. has more detailed info on the O Antiphons here.
After reading the O Antiphons, you see the significance of a familiar hymn:
Come, O come, Emmanuel,
and ransom captive Israel,
that mourns in lonely exile here
until the Son of God appear.
Chorus: Rejoice! Rejoice!
Emmanuel shall come to thee, O Israel!
O come, Thou Wisdom, from on high,
and order all things far and nigh;
to us the path of knowledge show,
and teach us in her ways to go.
O come, o come, Thou Lord of might,
who to thy tribes on Sinai's height
in ancient times did give the law,
in cloud, and majesty, and awe.
O come, Thou Rod of Jesse's stem,
from ev'ry foe deliver them
that trust Thy mighty power to save,
and give them vict'ry o'er the grave.
O come, Thou Key of David, come,
and open wide our heav'nly home,
make safe the way that leads on high,
that we no more have cause to sigh.
O come, Thou Dayspring from on high,
and cheer us by thy drawing nigh;
disperse the gloomy clouds of night
and death's dark shadow put to flight.
O come, Desire of the nations, bind
in one the hearts of all mankind;
bid every strife and quarrel cease
and fill the world with heaven's peace.
Hymn lyrics from fisheaters.