December 21st is the Winter Solstice which marks the shortest day of the year. While it actually is only momentary, from this point forward, we start to gain more daylight. It should not surprise us then, that the Church links this astronomical phenomena with the spiritual reality of Jesus, the Light of the World (John 8:12).
Prior to the Julian Calendar, December 25th marked the Winter Solstice which would also help explain why the Solemnity of the Incarnation from antiquity has been celebrated on this date. Once the calendar changed, Tradition continued the celebration on December 25th while the Solstice was recalculated to December 21st.
Once the O Antiophons became part and parcel of the Church’s liturgical celebration, O Oriens or O Radiant Dawn was established on the Solstice to continue to emphasize the point that,
The light shines in the darkness, and the darkness has not overcome it. (John 1:5)
Interestingly, the word for overcome is καταλαμβάνω which literally translated is comprehended. Light has also been a common symbol for knowledge. I believe the more literal translation provides a better explanation why the world does not recognize the Messiah. Darkness is the lacking of the knowledge of Jesus Christ. May we use the last days of Advent to evangelize our family and friends in order to take advantage of all the Christmas grace.
O Radiant Dawn, splendor of eternal light, sun of justice: come, shine on those who dwell in darkness and the shadow of death.” Isaiah had prophesied, “The people who walked in darkness have seen a great light; upon those who dwelt in the land of gloom a light has shown. (9:1).