The governing body of sumo—the Japan Sumo Association—organizes six tournaments annually: three of them are held in Tokyo in January, May, and September; one in Osaka in March, one in Nagoya in July, and one in Fukuoka in November.
Each sumo tournament or basho lasts for 15 days, beginning and ending on Sundays. When attending a sumo basho you can sit in a modern chair or opt for traditional Japanese-style floor seating. There are Japanese-style box floor seats or ring-side floor cushion seats. You can pick a traditional option if you don't mind sitting for a few hours without back support.
You can learn more about the sport if you see one of the sumo exhibitions in between sumo tournaments. You can also visit a sumo heya, where wrestlers train and stay. Visiting a sumo beya or a morning practice is also a good idea.
One interesting detail to note in sumo is that there is no weight restriction nor divisions meaning that someone smaller may end up fighting someone bigger. Seeing as how the whole point is to get someone to the ground or out of the ring this can certainly make for some interesting dynamics.
Don't miss your chance to see this ancient sport and its traditions in person!