No one knows when Mary is supposed to have given birth to Jesus, and for the first 300 years Christianity existed there was no celebration of the birth of Christ at all. If anyone wanted to celebrate or to mark the occasion, there was a tendency in the beginning, to lump his birth in with the Epiphany, an already established church feast which is held on January 6th .
Some church leaders even argued against celebrating Jesus' birth because it reminded them of the way Egyptians and other religions honored their pagan gods. In other words it was a pagan tradition to mark a deity's birth with celebration. Still, some church leaders argued that it should be celebrated and take place on May 20th. Other dates in May, November, January, and March were also championed.
March 25th was favored because it was considered at the time to be the day of the creation of the Sun according to the gospel.
December 25th gained ground in 273AD, and eventually stuck for some very non christian reasons. Simply put, it was a time in the year when people weren't killing each other, you know, burning christians at the stake or throwing pagans into the lions den, depending on which side of the religious fence you found your village.
The reason nobody was being sent to their god, was that there were two other holidays celebrated on this day. The Romans marked “natalis solis invicti” which means birth of the unconquered sun, and the Iranian holiday, the birth of Mithras or “Sun of Righteousness”, which was very popular among Roman soldiers.
Winter Solstice, yet another pagan celebration also occurred at this time, within a few days of the other two celebrations. So since nobody was fighting at this time, wiley church leaders decided to co opt the day, December 25th for their celebration of the birth of Jesus.
But Christmas didn't really catch on until many years later when Roman Emperor Constantine declared Christianity the number one religion of the land. The eastern parts of the empire, however, stuck with January 6th as the day to celebrate his birth and baptism. But eventually everybody with the exception of the Armenians moved to the December 25th date by 336AD. Western Churches still celebrate January 6th as the day of the arrival of the Magi instead of as Christ's baptism date.
That's not all, just about everything connected to Christmas is pagan in origin. For instance, gift giving and partying dates back to Roman Saturnalia, christmas trees with lights and giving to charity date back to the way the Romans celebrated their Roman New Year, and the Yule log as well as stuffing your face with all kinds of foods and drink harken back to Teutonic feasts, leading some super christians to contend that Christmas is nothing more than gift wrapped paganism.
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