Going to make a blanket statement here, but the Japanese seem to LOVE holidays. Christmas, New Year's, Valentine's... heck, there is a Saint Patty's Day parade here in Nagoya this weekend. If your culture has a holiday, the Japanese will adopt it and celebrate the crap out of it. I fully support this, for what it's worth.
On Valentine's Day here (and in South Korea, China, and Taiwan), it's a bit different than the States: Women buy men presents. Men are - for that day - totally off the hook. You get chocolates or sweets for husbands and boyfriends (or both, for the busier and more scandalous out there). These are called homei-choco (本命チョコ), which means chocolate of love. Coworkers and friends also get goods, which are called giri-choco (義理チョコ) or courtesy chocolates.*
I was expecting to find a long history of this holiday, given how old Japanese culture is and how they appreciate the past. Jokes on me: the holiday began in 1978 as part of a marketing strategy by confectionary company. It's nice to know Hallmark holidays are concept that extends beyond the US's borders.
* Gosh, I love the blunt and accurate descriptor of 'courtesy chocolates.'
** Wish I knew about the three fold thing a bit sooner. Would have changed my present selection, ifyouknowwhatImean.