25th December
The date came about due to a Roman tradition to celebrate the sun called
Saturnalia as part of the winter solstice during December. The 25th was
Dies Natalis Solis Invicti (the Birthday of the Invincible One). People decorated their houses & exchanged presents.
When Constantine became the first Christian emperor, he declared that it would be the day that Jesus' birthday would be celebrated instead.
The non-Christian Romans didn't mind as they still had an excuse to party!!

More
facts to follow..................................