Hōonkō (報恩講) is a holiday in the Jodo Shinshu tradition of Buddhism which commemorates the death of its founder, Shinran Shonin.
The observance began after Shinran's youngest daughter, Kakushinni (覚信尼, 1224-1283), assumed administration of Shinran's mausoleum, a duty later inherited by her descendants, who ultimately became the Monshu of Jodo Shinshu.
Followers will sometimes observe a strict diet that day, preferring to eat shōjin ryōri or "Buddhist cuisine", though this is entirely optional.
In the first decade after the death of Shinran, his grave in the eastern hills of Kyoto known as Otani (now Higashiyama-ku) was marked by a simple stone monument with the six kanji of the nenbutsu.
[1] The new location in Otani was further west, near the original grave of Hōnen, and a small hexagonal chapel was built on land donated by Kakushinni that had belonged to her deceased husband.