2009-05-04. I photographed this in the vicinity of Takadabashi of the Sagami River. At the time of photography, I was taking the seat next to the driver.
No.22 Koinobori Swim in Sagami River. (第22回泳げ鯉のぼり相模川) (Japanese sentence)
Children's Day takes place on May 5. Landscapes across Japan are decorated with koinobori from April to early May, in honor of sons and in the hope that they will grow up healthy and strong. (It is quoted by Wikipedia)
Koinobori; 鯉幟; こいのぼり > here (English WIkipedia)