Sapientiatide, or the seven days of the Great "O" antiphons, begins at Vespers this evening. The illustration is of the chant notation for the Seventh, sung on the 23rd of December. These "O" Antiphons inspired the great Christmas hymn, "O Come, O Come, Emmanuel", with the title taken from this Antiphon. In the original Latin, the hymn had seven verses one for each antiphon and, indeed, some good English hymnbooks have all seven. The usual English translation is by the Anglican, John Mason Neale.
The article from Medieval Saints at Yahoo!, with all seven antiphons in Latin and English.
The 'O' Antiphons in Rev. E. J. Quigley's The Divine Office , with a tip of the Red Beret to in illo tempore.