While I saw Tokyo's sumo arena out of sumo season, I stumbled on an active sumo tournament last summer. July is the month of the Nagoya Basho, and the flags of the various participating sekitori (officially ranked wrestlers, all the names on the flags ending with the character zeki, 関) welcomed not only the spectators, but also the visitors to Nagoya Castle. That's quite the entrance!
I did see one or two sumo wrestlers out and about, and made nothing of it until I noticed the flags. That's when I put everything together regarding what a man at the subway had asked. "Sumô? Sumô?" That's literally all he said, no other attempt to clarify. Yes, I know what sumo is, but it seemed unlikely to me that was what he was talking about - we were, after all, just standing in the subway tunnels waiting for a heavy shower to pass, the topic didn't fit the context and what I knew, I was just going to the castle. So I just stuck to looking a bit dim, like I didn't understand (which, to be fair, I was, and didn't).
As a footnote, the Nagoya Basho will no longer be held inside the castle walls from 2025. It moves to a brand new arena just to the North.