"Jesus’ birth celebration was deliberately set at the time of pagan feasts. They claimed that because the early Christians didn’t know when Jesus was born, they simply assimilated the pagan solstice festival for their own purposes, claiming it as the time of the Messiah’s birth and celebrating it accordingly." Some people say January 6, April, or even March. But it most definitely was not on Dec. 25.
http://www.biblicalarchaeology.org/daily/biblical-topics/new-testament/how-december-25-became-christmas/
http://www.biblicalarchaeology.org/daily/biblical-topics/new-testament/how-december-25-became-christmas/