(In response to Ken Shepherd's comments below about the "Messiah" being fine for Christmas.)
Ken, we're running into one of the big differences between the liturgical and non-liturgical Christian traditions (and no matter how much Evangelicals like to deny it, Evangelicalism is as much a tradition as Roman Catholicism.)
You say, "...Christianity is not about cycles, it's about the linear unfolding through time of God's eternal plan towards the end of this age and the inauguration of a new heaven and a new earth." In the Catholic view, the Christian life has a very strong cyclical component. We celebrate the birth of hope in winter, as the days begin to lengthen; we do penance before Spring so we can celebrate the resurrection of Christ.
To the Catholic, there is nothing wrong with following the cues of nature as we live out our baptismal vocation. Nature was created good by God, and we must adapt our lives to it even in the modern world. Those two facts urge us not to keep a constant focus on one aspect of the Christian mission -- spreading the word of God -- to the exclusion of others. Sometimes we should preach Jesus Christ, and sometimes we should merely adore and worship Him. The remembrance of his birth is an occasion for the latter.
Evangelicalism, as the name implies, has an omnipresent emphasis on conversion and downgrades adoration to a lower priority. What about the prophetess Anna, who "never left the temple but worshiped night and day, fasting and praying" (Lk 2:37)? Does the Evangelist imply that she should have been out proclaiming the Word, a la John the Baptist? No. She was living out her vocation: to pray for her fellow men and to adore God in the one place in the world where he dwelled in a special way. Clearly her example is one way to live out the Christian life.
To everything there is a season. There is a time for the Cross; but for now, it's still off in the distance. This is the season to kneel at the Christ child and be astonished once again that God wanted to become one of us.
Your points are noted.
I don't deny there's a time and season for everything.
Certainly I'd get annoyed if there was an altar call at every sermon in my church or if every message was on salvation.
Clearly the Bible teaches on a wide variety of subjects and clearly the teaching in churches is mostly geared towards preaching to the converted, literally, for training and instruction in holiness.
That being said, I think its crucial to make most of the days, for they are evil, to preach the Gospel, to evangelize, at times like Christmas and Easter. This does require we give full context of Christ, including the fullness of His nature and His mission.
I would say it's all subject to how the Spirit leads and to the nature of the makeup of the Christmas services.
Suppose for example a church's pastor notices that there are a lot of new faces in the pews leading up to Christmas, families and friends, perhaps of current parishioners in good standing. Certainly you'd gear your worship and your messages towards evangelizing the lost.
But if, on the other hand, the pews are filled with the usual suspects, you can focus more on adoration of Christ and on reinforcing and encouraging the faithful with teaching the Gospel in light of commemorating the Incarnation of Christ.
But no, I'm not solely about evangelizing 24/7. I see the propriety and the necessity of adoration, of prayer, intercession, fasting, worship, and Christian fellowship. Sometimes I think evangelicals jump headlong into evangelizing without proper preparation of prayer, fasting, and meditation on the Scriptures. Jesus fasted 40 days and nights before beginning his ministry. The Church fasted and prayed for 10 days from Christ's ascension to the Pentecost, when the evangelical mission of the Church was inaugurated as per Christ's prophecy and instruction (stay in Jerusalem until you are clothed from power from on high). By contrast, some evangelical churches in all sincerity evangelize without the proper timing, God's timing, and without proper spiritual prepartion. That criticism is well taken and agreed to.
Another minor point in response to the example of Anna the prophetess:
Scripture does teach that there are different parts of one body in the church, different ministries, different anointings of service, but one Spirit, one baptism, and one church body, headed by Christ.
As such:
Ephesians 4:11: And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers;
So I acknowledge some are given fully to evangelizing while others have prophetic ministries and others have pastoral or teaching ministries and others are prophets.
God motivates and inspires and anoints different people to different roles within the Church but all to the same end, raising people to a fuller maturity in Christ, starting from conversion to the faith through natural death. But of course, part of that grand mission does involve worship and adoration and fasting and praying, all exercises in and of themselves which don't evangelize per se but do gird us spiritually for the purpose of winning the lost to Christ and discipling them in the faith.
One other tiny minor point and I concede this: I am a bit disappointed with how very untraditional my church is at Christmas time. We do very little Christmas music in our praise and worship and I wish we'd do more of it, especially to bring us to a worshipful mood of adoring our Lord and remembering His incarnation.