If my memories serve me correctly (been quite a few years since I lived on the other side of the Pacific), White day is a totally made up holiday which serves as a "Men's Valentine's Day."
Basically, on Valentine's day, men are supposed to give women chocolate. Well, what if there's a guy who's a sweet tooth? No worries, because women are supposed to return the favor on White day, which is on...March 11th, I think? Hence, the "time period of chocolate."