May 5th is the Boy's Festival. (Usually May 5th is called "Kodomo no hi" (こどもの日 Children's day).)
Just as the girls of Japan have their own festival on the third day of March, the boys celebrate theirs on this day. This is called "Tango-no-Sekku" in Japanese.
On this day, miniature suits of armour and dolls representing warriors and legendary heroes are displayed in the house and huge cloth or paper carp called "koinobori" are flown from tall poles over the rooftop. Since the carp is regarded as a courageous fish because it swims upstream past waterfalls and many other obstacles, boys should follow its example by growing up strong and healthy, and rising to greatness by overcoming all difficulties.
(Quoted from "A bird's-eye view of Japan".)