A kind man from a different branch of the Christian family tree (and I don't say "kind" as a mere courtesy - this guy prays for me all the time despite his very busy life) emailed and said,
A few years ago, I saw a TV show that showcased this professor from Rutgers who studied up the nativity. He claims that Jesus was born on April 17, 6 B.C.E. or something like that. It gave me a case of the "Christmas blues". But last year, the Holy Spirit revealed to me that perhaps God wanted us to celebrate the Light of the world at the darkest time of the year.
This is where the church's liturgical calendar is such a treasure. Yes, in some cases (many Saints' days, for example) it observes verifiable historical dates. But, most of the time, the liturgical calendar is walking us through a constant celebration of faith-claims about Jesus.
The date of Christmas is a great example. The Biblical text itself gives little to suggest a "cold winter's night." It is very hard to argue for December 25th as the literal, historical birthday of Jesus.
What we celebrate at Christmas is John 1:14 - And the Word became flesh and dwelt among us, and we beheld His glory, the glory as of the only begotten of the Father, full of grace and truth. It is the fact of the Incarnation that we proclaim, and it is true on December 25th or April 17th or any day. In fact (as with so much of the Bible) the most important point can't be verified or disproved by historical investigation. Even if we knew the exact date of Jesus' birth, it would be secondary to our faith claim - that the unique Son of God took on our human nature in order to be the perfect sacrifice for the sins of the world.
My friend also mentions another spiritual theme - that of the Light shining in darkness (John 1:4-9). December 25th was a well-established pagan festival to cheer up folks in the darkest days of winter. Rather than try and beat the celebration out of existence, Christianity responded to a natural need for reassurance and brought in the spiritual truth of Christ, the Light of the World.
The church calendar keeps us walking through the whole message of Christ over and over again (just the way the Bible should be read - there's always something new). The Scriptural message is not a list of religious factoids and trivia to memorize. It is the source of our constant "participation" in the life of Jesus Christ.