O Radix Jesse.

The seven so-called "O Antiphons" (also known as the Greater or Major Antiphons) are sung at Vespers during the 8 days before Christmas Eve, a time known as the "Golden Nights."

Each antiphon begins with "O" and addresses Jesus with one of his titles which comes from the Prophets Isaiah and Micah. The titles' initials, when read backwards, are ERO CRAS. "Ero Cras" is Latin for "Tomorrow I come."

The titles for Christ are:

12/17: Sapientia (Wisdom)
12/18: Adonai (Lord of Israel)
12/19: Radix Jesse (Root of Jesse)
12/20: Clavis David (Key of David)
12/21: Oriens (Dawn)
12/22: Rex Gentium (King of the Gentiles)
12/23: Emmanuel (God is with us)

Here is today's antiphon:

O Radix Jesse, qui stas in signum populorum, super quem continebunt reges os suum, quem Gentes deprecabuntur: veni ad liberandum nos, jam noli tardare.

O Root of Jesse, which standest for an ensign of the people, at Whom the kings shall shut their mouths, Whom the Gentiles shall seek, come to deliver us, do not tarry.

Click here to hear it chanted, as it will be at Vespers the world over tonight.