There are many different traditions and theories as to why Christmas is celebrated on December 25th. 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! March 25th was also the day some early Christians thought the world had been made, and also the day that Jesus died on when he was an adult.