you could have just typed "wikipedia white day" into your address bar:
"In Japan, Valentine's Day is observed by members of the female gender who present chocolate gifts (either store-bought or handmade), usually to a member of the male gender, as an expression of love. On White Day, men who received a honmei-choco chocolate on Valentine's Day are expected to return the favor by giving gifts, usually more expensive. Sometimes the term sanbai kaeshi (三倍返し, sanbai kaeshi? 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."