User blog comment:Jake R/Monthly Blog- December 2011/@comment-1636990-20111202113639/@comment-1874924-20111202225615

Christmas is definitely not just for Christians. When the Roman Catholic church was in its youth, it fixed 25 December as the Feast Day Of the Nativity in order to piggyback on existing pagan Winter Solstice celebrations. This had the benefit of making Christianity more palatable to said pagans, but the price was that the Church has never had complete control over the holiday. In any case, the birth of Christ was a secondary matter to the early Church--belief in the Resurrection was really what marked a person as Christian.

The TV Tropes site has an interesting page on how the Japanese observe Christmas. Since there are few Christians in Japan, Christmas there is entirely commercial.