I have not heard this advice-- but perhaps it is because the fish might not have been delivered to the store for a day or two over the weekend. ??
When I buy fish, I almost always cook it the same day. Fish needs to be fresh and it can go bad on you in a short period of time. I can usually recognize fresh fish by the look-- and I also ask when it came into the store. I buy it at the fresh butcher counter, not pre-packaged.
At many restaurants they may be using frozen fish, but if it is an eating place that specializes in sushi or seafood it is probably fresh-- their reputation depends on it.