official sumo tournament; Grand Sumo Tournament
Refers specifically to one of the six annual professional sumo tournaments sanctioned by the Japan Sumo Association. Often used in contrast to exhibition tours (花相撲) or other unofficial events.
Next month's official sumo tournament will be held in Nagoya.
At the Grand Sumo Tournament, you can see the yokozuna's ring-entering ceremony.
本場所 is the general term for any official professional sumo tournament, while 春場所 names specifically the March tournament held in Osaka.
Compound of 本 (hon, 'main, real, official') and 場所 (basho, 'place, venue'). The term distinguishes official tournaments from other sumo events.