I'm sure you knew it already, But Jesus wasn't actually supposedly born on December the 25th.
It's been estimated that he was actually born some time in september.
The reason we celebrate it on the 25th of december, is a melding of pagan and christian beliefs. The 25th of december was actually the date of a pagan festival, and the christians decided to improve relations with the pagans by celebrating Jesus' birth on the 25th of december. Since then, lines have gotten blurry, and with the downfall of paganism and the uprising of Christianity, It soon became just the celebration of Jesus' birth.