Why do the Japanese celebrate Christmas?

For most countries, Christmas is like Valentine's Day. There's no real meaning or historical value ... it's just a hallmark holiday and an excuse to get into the theme of things, shop, and give/receive presents. Japan especially loves themed anything! A friend of mine recently converted to Islam but she still wants to celebrate Christmas ... although the holiday is technically "against her religion" she considers it more of an American tradition rather than a religious holiday.