I read somewhere that no one knows his exact date of birth. But you know, people like to celebrate something and tend to praise their Gods and saviors. I think that people wrote that Jesus was born on December 24. Also, the evening before Christmas in Catholic countries is on 24th December whereas in East Ortodox countries the evening before Christmas is on 6th January. I think no one knows the real date of his birth.
According to what I know, Jesus was not born on December. It is only declared December 25 because of the cold weather but their is no proof that it is really that day. We can't also find the exact date on the bible because what is in there are the description of weather and the surroundings.