The Commencement of the Great Antiphons (Dom Guéranger)
"The Church enters today on the seven days which precede the Vigil of Christmas, and which are known in the liturgy under the name of the Greater Ferias. The ordinary of the Advent Office becomes more solemn; the antiphons of the psalms, both for Lauds and the Hours of the day, are proper, and allude expressly to the great coming. Every day, at Vespers, is sung a solemn antiphon, consisting of a fervent prayer to the Messias, whom it addresses by one of the titles given Him in the sacred Scriptures."
"In the Roman Church, there are seven of these antiphons, one for each of the greater ferias. They are commonly called the O's of Advent, because they all begin with that interjection...."
Since I have been singing Gregorian chant for over two years now, and I'd like to forward its cause, here are the words and chant notation for today's antiphon, O Sapientia.
O Wisdom, that proceedest
from the mouth of the Most High,
reaching from end to end mightily,
and disposing all things sweetly!
Come and teach us the way of prudence.