The Japanese celebrate St. Valentine's Day in a rather unique fashion. Women give the men gifts of chocolate as well as other gifts.
It is said that St. Valentine's Day was imported to Japan in 1958 by a Japanese confectionery company
In 1960, Morinaga, one of the biggest Japanese confectionery companies, originated the present custom that only women may give chocolates to men.
In Japan, Valentine's Day is observed by females who present chocolate gifts (either store-bought or handmade), usually to a male, as an expression of love. The handmade chocolate is usually preferred by the receiver, because it is a sign that the receiving male is the girl's "only one"
These gifts of chocolate are divided into two types: giri choco (obligatory chocolate) and honmei choco (chocolate for the man the woman is serious about). Giri choco is given by women to their superiors at work as well as to other male co-workers. It is not unusual for a woman to buy 20 to 30 boxes of this type of chocolate for distribution around the office as well as to men that she has regular contact with.
Needless to say, the approach of Valentine's Day is something that department stores and shops look forward to and promote with zeal because of its potential for increased sales. Large displays featuring chocolate usually with heart-shaped displays start to grace the floors of department stores from mid-January or so.
A woman will normally purchase boxes of giri choco [chocolate of courtesy] in the several hundred yen range and may purchase an expensive box of honmei choco [chocolate of love] and another gift such as a necktie for her "special someone".

While all of this may seem quite one-sided, confectioners in Japan - never ones to miss an opportunity to sell more - took advantage of the Japanese feelings of obligation and created "White Day" in 1980 to help assuage the guilt feelings of those poor obligated males who received chocolate on Valentine's Day.
White Day was first celebrated in 1978 in Japan. It was started by the National Confectionery Industry Association as an "answer day" to Valentine's Day on the grounds that men should pay back the women who gave them chocolate and other gifts on Valentine's Day. In 1977 a Fukuoka-based confectionery company, Ishimura Manseido , marketed marshmallows to men on March 14, calling it Marshmallow Day.
Anyway, for the ladies the pay-off comes a month later on March 14, when it's the guys' turn to give something back. With the name White Day, I suppose the gift should be something white. On March 14th (White Day), exactly one month after Valentine's Day, the converse happens: males who received a "honmei-choco" [chocolate of love] or "giri-choco" [chocolate of courtesy] on Valentine's Day are expected to return the favor by giving gifts, usually more expensive. Sometimes the term sanbai gaeshi (literally, "thrice the return") is used to describe the generally recited rule that the return gift should be two to three times the cost of the Valentine's gift. Again, the stores provide plenty of reminders of the approach of this day so that even the most forgetful man cannot say that it slipped his mind. The gifts of chocolate that men buy are in white boxes (after all, it is "White Day") and come with separate shopping bags to put them in. If the gift is to be seen as really heartfelt, it needs to be handmade. So department stores will do a brisk trade in the various ingredients needed to make chocolate confectionaries. With all the romantic hype in stores and the media, as well as the pressure of giri-choco, a guy who gets no chocolates on Valentine's Day is entitled to feel like something is seriously wrong with not just his social life but also his workplace.
White Day, as you have probably already guessed, is a creation of marketing minds rather than anything remotely traditional. Though some sources talk about similar ideas in the mid-1960s, it's popularity only dates back to the early 1980's and so it doesn't have quite the marketing punch of its February partner. But if you got something from a young lady last week, you might want to consider the consequences of not giving something back...

Black Day (April 14) is a South Korean informal tradition observed for those sharing singleness for single people (a.k.a. Unit Solo/Solo Regiment, lee:) to get together and eat Jajangmyeon (noodles with black bean sauce), sometimes a white sauce is mixed for those who did not celebrate White Day.
The idea is that those who did not give or receive gifts on Valentine's Day or White Day can get together and eat Jajangmyeon, white Korean noodles with black bean sauce (hence the name), to commiserate their singledom.