Why December 25th?

In the mid 4th Century the Church in Rome chose the 25th December as a day of celebration to cooincide with a Roman midwinter festival Saturnalia. It also fell around the time the Pagan's celebrated the Winter Solstice. However before the date was set the Nativity would have been celebrated on Twelth Night (6th January) at the same time as one of the earliest Christian festivals Epiphany.

So there you go!