'Naked festivals'. A general term for popular festivals, mostly held at New Year in the coldest season, which these days feature near-naked young men usually dressed only in a fundoshi or mawashi (loincloth). Hadaka matsuri are often an opportunity for youths to show off their strength and manliness by jostling (see Dorokake matsuri), climbing or fighting over a trophy of some kind such as a wooden or straw ball, being sprayed with water or immersing themselves in a river. Similar hadaka matsuri are held at both Buddhist temples and Shinto shrines. A celebrated hadaka-matsuri is held for example at Enzo-ji Buddhist temple, Fukushima on January 7th. When the temple bell sounds at 8pm semi-naked youths and men swarm up a rope to the roof to attract good fortune for the coming year. At the Oni-jinja, Aomori, youths wearing mawashi make offerings of shimenawa at the shrine on new year's day. At the Kompira jinja, Akita, participants dressed only in a koshi-mino, grass underskirt, immerse themselves in the river for purification before offering candles and other gifts to the shrine, while at the Chokoku-ji temple in Nagano the procession includes mikoshi modelled in the form of sacred horses, bales of rice or sake barrels, and the participants jump into the river.
A Popular Dictionary of Shinto. Brian Bocking.