The fifth day of the fifth month was traditionally called Tango no Sekku and was a festival for boys.
On Children's Day, families with boys fly huge carp-shaped streamers (koinobori) outside the house and display dolls of famous warriors and other heroes inside. The carp was chosen because it symbolizes strength and success; according to a Chinese legend, a carp swam upstream to become a dragon.
In recent years, as more people have moved into apartments and smaller houses, the carp streamers have also gotten smaller, and there are now miniature versions that are decorated indoors.
Also on this day, families often take baths sprinkled with iris leaves and roots. This is because the iris is thought to promote good health and ward off evil.