At Vespers on these days, we pray one of the "O" antiphons at the Magnificat. Today, December 21, the antiphon is "O Oriens":
O Óriens splendor lucis æternæ, et sol iustítiæ: veni, et illúmina sedéntes in ténebris, et umbra mortis.
O Dawn, Reflection of eternal Light and sun of justice; come, and give light to those who sit in darkness and in the shadow of death.
From "Liturgical Meditations for the Entire Year" by the Sisters of St. Dominic, Adrian, MI (B. Herder, 1960):
As the dawn of Christ’s nativity approaches, Holy Church cries out for His everlasting brilliance to disperse the shadows of our misery and sin. Frequently in the Advent liturgy she speaks of Him as light to honor the splendor of His eternal divinity and the enlightenment of all men by His coming. Today she calls upon Him as the dawn, the first appearance of celestial brilliance upon the dark horizon of our world.
The splendor that streams from Bethlehem’s cave is meant to cast upon our lives the radiance of eternal light. Let us ask ourselves whether we shrink from this divine illumination. Perhaps there are dark corners in our souls, lurking shadows of sin, cherished spots of blindness where God’s light is not permitted to enter. If we allow the holy Infant only partial entrance to our thoughts and affections, He cannot dispel the darkness of our souls and lead us to everlasting light.