Actually, I did mean the training/practice.
Usually, it's a 6am or 7am start for this. Apparently, there are Beya/stables that allow visitors access during tournament season - this might be something to talk with a hotel concierge about.
http://www.japantimes.co.jp/sports/2009/08/20/sumo/how-to-view-sumo-up-close-and-in-person-asageiko/#.V1Gi8Ym9Kc1