White Day is a holiday on March 14th where a boy must return the favor to the girl that gave him chocolate on Valentine's Day. Also known as "Reply Day".
White Day is a created holiday celebrated in Japan that is connected to Valentine's Day. It is held one month after Valentine's Day on March 14. A day when men are to reply to the gifts they are given on Valentine's.
White Day was originally known as Marshmallow Day back in 1977. It was so named because of a Japanese confection company called Ishimura Manseido that was marketing marshmallows to men. It became an official holiday by the name of "White Day" a year later in 1978 by the National Confectionery Industry Association of Japan. It was designed to be a day when men were to answer or reply to the girl that would give a man chocolate on February 14th.
This normally applies to those men who are given the type of chocolate referred to as Favorite Chocolate (honmei-choko). This is a type given when a girl has a more romantic interest in the male. The Obligation Chocolate (giri-choko) is more given as a polite gesture to co-workers and doesn't require any reply for White Day.
Common gifts for men to give a woman on White Day includes white/dark chocolate, marshmallows, jewelery, and white lingerie or other kinds of clothing. Since in Japanese culture it's not common to see men actually purchasing lingerie from a store. More often these kinds of gifts are purchased through online retail stores.