Valentine's Day in countries which observe White Day is typically observed by girls and women presenting chocolate gifts (either store-bought or handmade), usually to boys or men, as an expression of love, courtesy, or social obligation.
On White Day, the reverse happens: men who received a honmei-choco (本命チョコ, 'chocolate of love') or giri-choco (義理チョコ, 'courtesy chocolate') on Valentine's Day are expected to return the favor by giving gifts. Traditionally, popular White Day gifts are cookies, jewelry, white chocolate, white lingerie, and marshmallows. Sometimes the term sanbai gaeshi (三倍返し, 'triple the return') is used to describe the generally recited rule for men that the return gift should be two to three times the worth of the Valentine's gift.
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, Ishimuramanseido, marketed marshmallows to men on March 14, calling it Marshmallow Day (マシュマロデー Mashumaro Dē).
Soon thereafter, confectionery companies began marketing white chocolate. Now, men give both white and dark chocolate, as well as other edible and non-edible gifts, such as jewelry or objects of sentimental value, or white clothing like lingerie, to women from whom they received chocolate on Valentine's Day one month earlier. Flowers and other gifts are also given on this day. Eventually, this practice spread to the neighboring East Asian countries of South Korea, China, Taiwan and Vietnam. In those cultures, White Day is for the most part observed in a similar manner.
As a note in Japan, if chocolate given to a man a month prior was giri choco, the man may not be expressing actual romantic interest, but rather a social obligation.