Koinobori are carp-shaped flags traditionally flown in Japan to celebrate Children's Day. Traditionally, the Japanese associate carp with boys because of the strength associated with carp that swim upstream. These flags are made by drawing carp patterns on paper, cloth or other nonwoven fabric. They are then allowed to flutter in the wind. They are also known as satsuki-nobori.
Koinobori flying in [Oboke Koboke], Iya Valley, Tokushima Prefecture
Koinobori flying in [Oboke Koboke], Iya Valley, Tokushima Prefecture

Children's Day takes place on May 5th. Landscapes across Japan are decorated with koinobori, or carp streamers, from April to early May, in honor of sons and in the hope that they will grow up healthy and strong. Often, the short Koinobori Song is sung by kids and their families.

These carps are flown on the rooves of houses with sons, with the biggest 'koinobori' for the eldest son, ranging down to the smallest carp for the youngest son.

The sizes of these 'koi flags' can be usually from a few inches long to a few meters long. In 1988, a koinobori with length 100m, and weight 350kg was made in Saitama.