I just read a wonderful and well researched account on how December 25th (or January 6 for old calendarists) came to be celebrated as the day of Christ's birth. It's well worth the read and instead of getting permission to reprint it, here's the link to it from BAR Magazine, Biblical Archaeological Review. Looks like they have lots of interesting articles and even some free e-books.
The picture above is an Orthodox icon for the Nativity of Christ. It is a more modern one. The one below is the more ancient one.
And remember, the 12 Days of Christmas begin on Christmas Day.