Sumo's 15-Day Tournament Was a Postwar Compromise

Professional sumo in Japan held two honbasho — grand tournaments — per year through the Edo period and into the early twentieth century: one in January in Tokyo, one in summer in Osaka. By the postwar period, the two main wrestling organisations, the Edo-based Nihon Sumo Kyokai and the Osaka-based Osaka Sumo, had merged under occupation-era pressure in 1925, but the scheduling of tournaments remained contested.

Through the late 1940s, each Tokyo honbasho ran for ten days. The Japan Sumo Association extended it to fifteen days in January 1949 after deliberation over how to better fill the Kuramae Kokugikan arena and generate revenue during a period of economic reconstruction. A Nagoya tournament was added in 1958, and that same year the final count of six annual basho — each fifteen days — was established.

The format distributes tournaments across the calendar: Tokyo in January, March, and May; Osaka in March; Nagoya in July; and Fukuoka in November. Each wrestler in the top division faces fifteen opponents over those fifteen days — one per day. Their win-loss record determines promotion, demotion, and in the case of yokozuna, the informal pressure to retire if losing records accumulate.

The fifteen-day structure means a yokozuna who wins every match across a tournament goes 15-0. The term for that perfect score is zensho yusho, and it has happened 83 times across recorded honbasho history — roughly once every few tournaments, not as rare as it sounds, but uncommon enough that each instance marks a career.