Hi Carol
Best time to go to the market is in the early morning, because usually the good sushi shops will form queue from 6am. But as I was with my kid, I ended up around 9am and the queue will be much longer, but still worth a try if you do not mind the queuing. (mostly an hour or so)
As for the other parts of the market, good to go before noon as well, because you can see most products there. Just roam around, there are many jems there.
I got here the calendar you can check out before you plan your itinerary. (Skip the red box / yellow box / red fonts dates, other than that you are safe ^^) Its only available in the Jp site.
p.s. I heard that they are going to demolish the place and make way for the Olympics 2020, so they will be moving off, good to go before that. 🙂
Cheers!
Dawn