From the Japan Sumo Association.
Sumo grew out of contests of strength and grappling. Similar sports have been played around the world since ancient times.
In Japan, its origins are traced to a myth of a contest of strength in the Kojiki (712) and the Nihon Shoki (720), and to the legend of a match between Sukune and Kehaya held before the Emperor.
Sumo was held every year as a festival ritual to tell the fortune of the year’s harvest. It later became a court event and went on for 300 years.
From the Kamakura period to the Sengoku period, warriors practiced sumo as training for battle. Oda Nobunaga loved sumo and held matches with wrestlers gathered from many places.
In the Edo period, some people made sumo their job, and sumo shows were held all over the country. By the middle of the Edo period, they were held regularly.
Three great wrestlers, Tanikaze, Onogawa and Raiden, appeared, and sumo was even held for the shogun to watch. This built the base of today’s grand sumo.
Today there are six grand tournaments a year: January, May and September in Tokyo, March in Osaka, July in Nagoya and November in Fukuoka.
There is no single right answer. Talking it over with someone at home is a good way to learn.