There is trend the company use oversea event to own marketing for selling own product. Christmas, Valentine and Halloween. for example. Japanese people are influenced by them promotion around Japan for long time and became don't have doubt for participation to these events without faith. Many Japanese are thinking it is Japanese style.
As @deshimaru said, it's a commercial event like Halloween in Germany but bigger, mainly thanks to Americans for bringing out so many movies about Xmas to brainwash them on how much money they have to spend to enjoy a good Xmas lol
Japanese culture has a strong polytheism background, so they are much less bothered about celebrating religious events from different religions as long as they have fun. It's interesting to see how whole town quickly switches from Christmas to Japanese New Years on the 26th, to get ready to pray for their Shinto gods just after celebrating the birth of Christ.
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