The people who had invented 25 of December as the day of the Christ birth were the same that the birth of the sun (in accordance with they it day that the sun starts a new cycle). The Bible does not say the date of its birth, but of the forts you evidence of that it was not in December. For example, this month is cold and rainy in the place where Jesus was born, the Bible says until it was impossible to be outside of house at this time. But it says that the shepherds lived to the outdoors, what he indicates that its birth did not occur in this month. Another one evidences is that Jesus died with 33 years and way. It died in one month that corresponds in our calendar to the months of March and April, if to count more 6 months, what he would correspond the one year more, will arrive at the months of September and October, the probable months of its birth.