A belief that certain days of the year are days of great misfortune. Koushin days appear six times within the year, as well as in the Koushin Year, the 57th year of the 60-year Zodiac cycle. Special Buddhist (originally Chinese Taoist) rituals are performed on these days to ward off evil influences, to protect against misfortune, and to cure illness.

The monkey is an important part of the rituals, since the "shin" (申) character also means "monkey" and is the Zodiac animal that is associated with Koushin days.

Shoumen Kongou, a god of healing, became associated with these rituals in the Kamakura Period in Japan.