It is said that most Japanese believe in Shintoism and/or Buddhism and only one percent of its population believe in Christianity.
But on December 24th and 25th, 90% or more act as if they were Christians. For many couples the most important day is the 24th . On the 24th, they go out together, for example, for dinner or to spend time together and exchange presents to confirm their relationship. For children, Christmas is an event in which they can wish for whatever presents they want. I believe, Santa Claus is the most important person for them. Some parents try to make their children believe that Santa Claus has really visited.
The most surprising thing to me is that on 26th of December, every single Christmas item is taken away and things for welcoming the new year will be prepared. If you are in Japan in December, you can feel this skittishness in your bones.