O Radiant Dawn, splendor of eternal light, sun of justice: come, shine on those who dwell in darkness and the shadow of death.— Great “O” Antiphon of the Canticle of Mary, for Vespers, on December 21st.
The ancient Church saw the ages of man before the coming of the Messiah as times of darkness, while our age, the age of the Church, is a time of daybreak — the light of dawn has appeared in the skies, and we face towards the east awaiting sunrise, which represents the Messiah's return and final glory. Ancient churches themselves were oriented towards the east, with priest and faithful all facing eastward towards the coming Lord. Facing the east is a custom largely lost in the Catholic Church, and many — including Pope Benedict — think that we ought to recover this symbol of our Faith.
The sun, in its course through the seasons, is a natural symbol for higher things, which the Church reflects in her liturgies. The end of Advent happens to occur around the time of the winter solstice in the northern hemisphere, which marks the shortest day of the year, and so is a fitting symbol of the new light of Christ coming into the world.
Many critics say this kind of nature symbolism is pagan, and that Christian celebrations ought not be tied earthly events. But this kind of spiritualism has no place in the Church on earth, for God made the world and it was very good. Indeed, many of the spiritualistic critics themselves often descend into paganism, worshiping the solstice itself instead of Him who Made it.