History and traditions of dating
Many Orthodox and Coptic Churches still use the Julian Calendar and so celebrate Christmas on the 7th January (which is when December 25th would have been on the Julian calendar).
But soon people wanted a separate day to celebrate his birth.
Some people also think that December 25th might have also been chosen because the Winter Solstice and the ancient pagan Roman midwinter festivals called 'Saturnalia' and 'Dies Natalis Solis Invicti' took place in December around this date - so it was a time when people already celebrated things.
The Winter Solstice is the day where there is the shortest time between the sun rising and the sun setting. To pagans this meant that the winter was over and spring was coming and they had a festival to celebrate it and worshipped the sun for winning over the darkness of winter.
A very early Christian tradition said that the day when Mary was told that she would have a very special baby, Jesus (called the Annunciation) was on March 25th - and it's still celebrated today on the 25th March.
Nine months after the 25th March is the 25th December!