This special event is occurs in March 14th, a month after of Valentine’s day in Japan.
In Japan, only girls are able to give flowers or chocolates to guys on Valentine’s day. In March 14th, this time guys need to give back something to girls. It could be anything but he has to give something all girls who gave him present.
This culture is made because there is “give back” culture in Japan. if someone receives a present from someone on special occasions, the person who receives has to give back something. This is the way of japanese people to keep harmony and fairness between people.
So that everyone feels same.