Unfortunately this festival has been cancelled permanently "due to issues such as aging of materials and equipments and changes in the environment of the venue in recent years."
"Koinobori" (鯉のぼり) are carp streamers that are hung around Japan in celebration of "Kodomo No Hi" (こどもの日) Children's Day. Children's day is celebrated on May 5th.
Since 1988, Sagamihara has had a festival dedicated to this day. To decorate for this festival, large ropes with 1200 carp streamers hung above the river. There are also traditional games, and food stalls selling typical Japanese festival food.
Where: 泳げ鯉のぼり相模川 〒243-0301 神奈川県愛甲郡愛川町252-0244
How much: Free
Website(Japanese): https://www.oyoge-koinobori.com/
great pics & info! too bad it's cancelled for good. lucky you got to experience it!