The early Church did not celebrate Christmas much -- this is true -- but that was because the focus was centrally on the resurrection of Christ from the dead (Read what Paul wrote to the Corinthians). This was the big deal -- dying and rising. Easter remains the Queen of Seasons even though the marketplace has not done to Easter what it did to Christmas. The date of Christmas was fixed not by pagan celebrations but by the passion and death of Christ. In the West the date calculated was March 25 (in the East they used and still use a different calendar system). March 25 was the first date fixed because at the time of Christ it was commonly held that prophets died on their birth or conception date. It’s the idea of “integral age,” as scholar William J. Tighe has noted in such detail. The Annunciation of our Lord to the Blessed Virgin Mary is liturgically celebrated on March 25, the date of Christ’s conception through the Word spoken by Gabriel and enacted by the Spirit. In addition, you can read of the theologically-important connection between the womb and tomb in the work of John Behr in The Mystery of Christ. So because Christ died on the same date of the Annunciation (his conception), then Christmas Day has to be exactly nine months later OR March 25.
But this is not the only reason to interrupt Lent with this wonderful day of rejoicing. For Blessed Mary is the first Christian (pondering all these things in her heart after consenting to the will of the Lord). She is our own best example of faith under fire, of trust where eyes and experience say "no". She is our mother in the faith and from her we learn what it means to believe the Word of the Lord (which came to her with more than an inconvenient message and one that challenged everything she had come to know and believe of life). On this day we rejoice to stand with her before her Lord and ours, in whom we have forgiveness, life, and salvation.
Let me close this day with a little paragraph from Augustine from On The Trinity:
For He is believed to have been conceived on the 25th of March, upon which day also He suffered; so the womb of the Virgin, in which He was conceived, where no one of mortals was begotten, corresponds to the new grave in which He was buried, wherein was never man laid, neither before nor since. But He was born, according to tradition, upon December the 25th.