Women buy men chocolates on Valentines day. The night before Valentines day — huge crowds of ladies descend upon Japanese department stores in a rush to buy chocolates for their sweethearts.
In many cases, women also buy chocolates for their boss, coworkers, brothers and father. This is known as "giri chocolate" — this can be roughly translated as "obligation chocolate". Women don't really have any interest in buying such gifts — they buy it out of a sense of obligation.
Men are expected to reciprocate with a gift one month later on March 14th (white day). If a girl is lucky she might get an expensive gift from her sweetheart on white day. If she's unlucky she'll get marshmallows.