I have a new category of links on the top of my sidebar: Advent and Christmas Spiritual Resources. The list is broken down into 5 different sites (I-V). Under Creighton.edu, the 10 letters (A-K) are specific links within that site. There are many others, so feel free to go and browse on all the links.
The virtue for December is Union with God. If you click on the picture of the Madonna and Child with the Holy Eucharist I have on my sidebar, it will take you to a sermon by Father John J. Lombardi, Chaplain of the National Shrine of Our Lady of Lourdes at Mount Saint Mary's University in Emmitsburg, MD on the subject of Union with God during an Advent Retreat.
Further down on the sidebar, is a picture (with a link) of the Immaculate Conception, the traditional devotion for December. The Feast of the Immaculate Conception is December 8, and is a Holy Day of Obligation. In spite of its occurrence during the season of Advent, the Immaculate Conception does not refer to Jesus' conception in the womb of Mary, but rather to Mary's conception, free from original sin, in the womb of St. Anne.
Thou art all fair, O my love, and there is not a spot in thee
~Canticle of Canticles 4:7
Advent is a wonderful time to meditate on the role of Mary in salvation history.