"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.
Nagabusa Koinobori Festival (長房ふれあい端午まつり)
Koinobori at Mine Hakusan Shrine
Sagamihara Koinobori Festival for Children's day