White Day was first celebrated in 1978 in Japan. It was started by the National Confectionery Industry Association as an "answer" 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, marketed marshmallows to men on March 14, calling it Marshmallow Day.
Soon thereafter, confectionery companies began marketing white chocolate. Nowadays, men give both white and dark chocolate, as well as other gifts, to women from whom they received chocolate on Valentine's Day one month earlier.
White Day is definitely a uniquely Japanese invention, but it’s spread to some other Asian countries, including South Korea.