Two different aspects of the Advent Season begin today, the Ember days in Advent and the first of the 7 Greater Advent Antiphons (December 17-23).
Today is Ember Wednesday in Advent. The other Ember days are Friday and Saturday of this week. These days come in the first week following the Feast of St. Lucy (or after the Third Sunday of Advent). The Introit for this day is the same as that for the Fourth Sunday of Advent (Rorate coeli) and the Gospel reading from Luke 1 helps to prepare us for Christmas.
The "O" Antiphon for December 17 is:
O Sapientia, quae ex ore Altíssimi prodiísti, attíngens a fine usque ad finem, fórtiter suavitérque dispónens ómnia: veni ad docéndum nos viam prudéntiae.
O Wisdom, which camest out of the mouth of the Most High, reaching from end to end and ordering all things mightily and sweetly: come and teach us the way of prudence.
Lutherans will find all of the "O" Antiphons listed in the Lutheran Service Book in connection with hymn 357, Veni Emmanuel