I received a postcard JP-396790 today, featuring koinobori (鯉幟 in Japanese), also known as "carp streamer". Those carp-shaped wind socks were historically flown by Samurai households in Japan during the Edo Period (江戸時代: 1603-1868) to celebrate Tango no Sekku (端午節), the 5th day of the 5th month in the lunisolar year, in hope that their sons would grow up healthy and strong. The tradition can be further traced to an ancient Chinese legend (鯉魚跳龍門) that carps in the Yellow River would become powerful dragons if they could leap upstream over a rough section of the river cascades in Longmen area.
As a result of switching to the Gregorian calendar after the Meiji Restoration (明治維新 or Meiji Ishin), the celebration takes place on May, 5th. Rooted in the Samurai culture, the celebration was known as Boy's Day since it was for boys only until the Children's Day was designated as a national holiday in 1948. The Koinobori sets are now flown for all children from April to early May.
As a side note, since carp streamers are also referred as koi streamers, I was wondering what the difference was between carp and koi. It turns out that koi are a domesticated, ornamental variety of common carp (Cyprinus carpio). Check out a couple of funny comments at Yahoo! Answers.