Though skeptics try at times, in the service of their own silly parallelisms, to imply that the December 25 date for Christmas is somehow essential to Christianity, most Christians know that this is not true. We don't actually know what time of year Our Lord was born.
But I think that those men who decided--whoever they were--to celebrate it in December were wise. Of course, they didn't foresee the spread of Christianity to the antipodes, where all is topsy-turvy and this time of year is warm, with long, sunny, days. Our brethren Down Under will have to forgive us for celebrating that whole, wonderful, awe-inspiring cultural structure that has grown up around the (probably deliberate) decision to celebrate the Feast of the Nativity at the time of the winter solstice in the Northern Hemisphere, the time of darkness and cold.
Many beautiful carols have arisen out of this choice, but one of my favorites is "Lo, How a Rose," and in all of it, my favorite line in the English translation is
She bore to men a Savior, when half-spent was the night.
The allusion here is double. First, the line alludes to the quotation from the apocryphal Book of Wisdom which is used in the introit to the Mass for Christmas:
While all things were in quiet silence, and the night was in the midst of her course, Thine almighty Word, O Lord, leaped down from heaven from Thy royal throne.
But the phrase also alludes, probably, to Romans 13:12,
The night is far spent, the day is at hand: let us therefore cast off the works of darkness, and let us put on the armour of light.
By a happy accident, if it was an accident, of the translation, one can take the statement that the night was half-spent to mean either that the night was half over or that the night was half exhausted.
The time, in our world and in our country, is not bright. We may indeed feel that we are deep in the night, that the night is half-spent, and that our civilization, too, is half-spent, exhausted, its vital force and will to live running out.
The message of Christianity, embodied in the choice of the darkest time of the year for the celebration of the birth of the Light of the World, is that it is at just those times that God acts. Ever since Adam fell mankind has been in darkness. But St. John tells us that the light shone in the darkness, and the darkness was not able to overcome it. Into this, our dark world, God sent His Son, made of a woman, that we might receive the adoption of sons.
Thanks be to God.