Valentine's Day and White Day
taken from Japan Related Information
The Japanese celebrate St. Valentine's day in a rather unique fashion. Women give the men gifts of chocolate as well as other gifts. 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 in the several hundred yen range and may purchase an expensive box of honmei choco 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. On March 14th, exactly one month after Valentine's Day, men who were lucky enough to receive gifts of chocolate have the chance to return the favor by giving the women who gave them gifts of chocolate a more expensive box of chocolate or sweets (for some reason or other, these return gifts seem to be priced slightly higher than those the women purchase). 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.
Relevance to Urusei Yatsura
Ryuunosuke, being the stud that she is always receives tons of chocolates on Valentine's Day from other girls. Mendo recieves a ridiculous amount as well, but nothing in comparision to Ryu's pile.