5th of May is the Children's Day in Japan and we put up Koinobori, the carp-shaped wind socks above the roof or across the river. It was originally only for boys so normally only families with boys put up the decoration. But it's a national holiday and everyone gets the day off!
There is a chinese tale about the dragon gate, which is a rapid stream along the Yellow River. If a fish can climb up the stream and go through the gate, the fish will become a dragon. And of all the fishes tried, a carp made it.
So we fly the carps to wish that our children will become dragons.