O

It's the shortest, but most expressive word in the Bible, and starting today it characterizes the liturgy. Every one of the seven days leading up to Christmas Eve features an O Antiphon: it appears in the Liturgy of the Hours as an antiphon and at Mass as the Alleluia Verse. The translations in the liturgy are pretty truncated, however, so you don't get the full sense of how much Scripture is packed into each of these invocations of the coming Lord.
Today's fuller antiphon goes something like this: O Eternal Wisdom, proceeding from out of the mouth of the Most High, who reaches from end to end and orders all things mightily and sweetly, come now to direct us in the way of holy prudence.
Part of the reasons these antiphons appear today is that the Advent liturgy shifts into high gear on December 17. Suddenly, we move from "generic Advent" to "intense and immediate preparation." Look at the readings for Mass: we have gone from general prophecies of a great figure, of peace, of consolation to today's "countdown to the King" in the form of Jacob's prophecy of kingship for Judah and Matthew's genealogy, tracing the bloodline of the kingship to David and then to Jesus Christ.
From now on, every day of Advent will show us the step-by-step fulfillment of the prophecies: the annunciation to Joseph; the announcement of John the Baptist's birth; the annunciation to Mary; the Visitation; the birth of the Baptist. (Isn't the Liturgy amazing?)