Originally Posted by Darth InSidious
It was more likely chosen since it was nine months after the feast of the Annunciation, which falls on March 25th. This in turn was probably calculated by using the references to Zechariah being in the Holy of Holies (Luke 1:8-25) - which would have happened at the Feast of Tabernacles. Jesus, Luke's Gospel also claims, was conceived in the sixth month of Elizabeth's pregnancy (Luke 1:26), placing his conception around the spring equinox.
See also: "De solstitiia", "De Trinitate" IV,5.
The 25th of December was Saturnalia among the Romans, who codified the dogma of what is now the Roman Catholic Church. Originally it was named after Christ, but anyone who has checked the weather reports of the Holy land will tell you that only someone interested in frozen mutton, and frozen shepherds would be 'in the hills watching the flocks by night', so he was not born then.
It originally was supposed to be a contemplative holiday like Yom Kippur, a time to be thankful for what God had given, and consider what you did wrong. However between around 400 AD and the 10th century, the holidays of Saturnalia and Yule infected them, and began the holiday we know now.
As for 'Old Saint Nick' He was a Bishop of Myra Turkey under Diocletion, one of the first Popes of the Eastern Orthodox church. He supposedly threw money in through the window, or down the smoke hole (The chimney had yet to be invented) so that three poor women had proper doweries.