I believe this claim may have originated from (or at a minimum was perpetuated by the book [Kitchen Confidential](http://en.wikipedia.org/wiki/Kitchen_Confidential).  Therein, then New York chef [Anthony Bourdain](http://en.wikipedia.org/wiki/Anthony_Bourdain) admitted that most fish suppliers in NYC are closed on Mondays thereby forcing restaurants to use older fish.  If you accept Mr. Bourdain's claim, then I think that confirms the claim.  That book was written a decade ago, so I am not sure if that is still universally the case in NYC.

Anecdotally, my local fishmonger (who is not located in NYC) admittedly is only able to receive deliveries of fresh fish every day *except* Wednesdays, so everything that is sold on Wednesdays is at least a day old.

Specifically related to *sushi*, however, one should note that many of the popular fish served in sushi restaurants, *e.g.*, large fish like tuna, is delivered frozen and will therefore not be as affected by delivery dates.

(I realize I am a bit short on citations here; I'm working on it and will edit as I find them.)