(wikipedia)

Koinobori (鯉幟), meaning « carp streamer » in Japanese, are carp-shaped wind socks traditionally flown in Japan to celebrate Tango no sekku (端午の節句), a traditional calendrical event which is now designated a national holiday; Children’s Day (Kodomo no Hi, 子供の日). These wind socks 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 (皐幟).

Children’s Day takes place on May 5, the last day of Golden Week, the largest break for workers and also a week in which businesses usually close for up to 9–10 days. Landscapes across Japan are decorated with koinobori from April to early May, in honor of children for a good future and in the hope that they will grow up healthy and strong.