Today, the shortest day of the year, the O antiphon is this:
"O Radiant Dawn, splendor of eternal light, and sun of justice, come, and shine on those seated in darkness, and in the shadow of death."
Christ draws near. He said of himself, "I am the light of the world."
The first letter of John simply says, "God is light."
Up to now darkness has been increasing every day. But with the solstice, each day will bring us a bit more light. The birth of Christ is celebrated at this time of year to show in a symbolic way that he is indeed the light of the world.
John the Baptist's birth is celebrated in the liturgy on June 24, near the summer solstice. Symbolically, it fits with what John said about himself: he is not the light, but the precursor of the light. "He must increase, and I must decrease," he declared. After John's birth the days begin to grow longer, but with Christ's coming we start to walk in greater light.