Christian Origins of Christmas
Around 220 AD, the theologian Tertullian declared that Jesus died on March 25, 29, but was resurrected three days later. Although this is not a plausible date for the crucifixion, it does suggest that March 25, nine months before December 25th, had significance for the church even before it was used as a basis to calculate Christmas. Modern scholars favor a crucifixion date of April 3, 33, which was also the date of a partial lunar eclipse (These are Julian calendar dates. Subtract two days for a Gregorian date.).
By 240 AD, a list of significant events was being assigned to March 25, partly because it was believed to be the date of the vernal equinox. These events include creation, The Fall of Adam and Eve, and, most relevantly, the Incarnation.
The view that the Incarnation occurred on the same date as crucifixion is consistent with a Jewish belief that prophets died at an "integral age," either an anniversary of their birth or of their conception.
The idea that December 25 is Jesus' birthday was popularized by Sextus Julius Africanus in Chronographiai (221 AD), an early reference book for Christians. This identification did not at first inspire feasting or celebration. In 245 AD, the theologian Origen denounced the idea of celebrating the birthday of Jesus "as if he were a king pharaoh." Only sinners, not saints, celebrate their birthdays, Origen contended.
Read more of the entry "Christmas" at Wikipedia.
As Constantine ended the Christian persecution and began the persecution of non-Christians, Christians began to debate the nature of Christ. The Alexandrian school argued that he was the divine word made flesh (see John 1:14), while the Antioch school held that he was born human and infused with the Holy Spirit at the time of his baptism (see Mark 1:9-11). A feast celebrating Christ's birth gave the church an opportunity to promote the intermediate view that Christ was divine from the time of his incarnation. Mary, a minor figure for early Christians, gained prominence as the theotokos, or god-bearer. There were Christmas celebrations in Rome as early as 336 AD. December 25 was added to the calendar as a feast day in 350 AD.