Professional sumo adopted six major tournaments per year in 1958.
Before the change, Japan held fewer official tournaments annually. The 1958 reform created the six-basho schedule that became the foundation of the modern professional calendar. Each tournament lasts 15 days, producing a regular cycle of competition in which wrestlers can rise or fall through the rankings.
The six tournaments are traditionally held in January, March, May, July, September, and November. Tokyo hosts three of them, while Osaka, Nagoya, and Fukuoka host one each. The November tournament is the final major event of the calendar year.
This schedule should not be confused with exhibition tours or regional jungyō events. Those appearances are official sumo activities but are not counted among the six ranking tournaments that determine promotion, demotion, and championship records.