I don't know why, but recently the fish section in local retail stores has started to focus on fish on Tuesdays.

My local AEON is selling fresh fish a lot more at relatively accessible prices on Tuesdays. Last week I bought four fat sardines from Toyama for 98 yen each and a locally caught (what we call) Arakabu for 498 yen, both of which were exceptionally fresh and nice compared to my recent purchases. 

Meanwhile, at Iwataya you can get 2 packs of department store quality (I don't mean sashimi quality, though) small cuts for 1,080 yen every Tuesday. But the variety tends to be quite limited to marlin tuna, red sea bream, Japanese Spanish mackerel (complicated...what we call Sawara)and Norwegian salmon. I usually end up with the salmon and mackerel.  

Daimaru also do a special fish sale on Mondays and Tuesdays where whole fish are sold at a 20% discount. However, as there is no list price when it comes to fresh fish, I am a bit suspicious of the prices, to be honest. But the quality and variety of the fish is, in my opinion, the best of these three. Only if the queues at the checkouts were not punishingly long and the shop assistants a little more attentive!