He's talking about restaurants in New York City, where he spent most of his culinary career.
The reason has nothing to do with religion, superstition, or the like. Instead, he says, it has everything to do with quality.
"I know how old most seafood is on Monday — about four to five days old!" he wrote. "Chances are good that that tuna you're thinking of ordering on Monday night has been kicking around in the restaurant's reach-ins ... co-mingling with the chicken and the salmon and the lamb chops for four days."
Disturbingly, fresh fish lasts only about three days tops, and that's only if you refrigerate it properly, according to New York city expert fishmongers interviewed by the Huffington Post.
See Bourdain through his career:
Chef Anthony Bourdain
World famous chef Anthony Bourdain refuses to eat restaurant fish on Mondays — and there's a very good reason why
EXCLUSIVE - Anthony Bourdain poses backstage with the outstanding informational series or special award for "Anthony Bourdain Parts Unknown" at the Television Academy's Creative Arts Emmy Awards at Microsoft Theater on Saturday, Sept. 12, 2015, in Los Angeles. (Photo by Rich Fury/Invision for the Television Academy/AP Images)
(AUSTRALIA & NEW ZEALAND OUT) Chef Anthony Bourdain, 27 October 2003. AFR Picture by MICHELE MOSSOP (Photo by Fairfax Media via Getty Images)
MIAMI BEACH, FL - FEBRUARY 26: Chef Anthony Bourdain does a cooking demonstration at the South Beach Food And Wine Festival on February 26, 2005 in Miami Beach, Florida. (Photo by Gustavo Caballero/Getty Images)
UNITED STATES - APRIL 12: Chef Anthony Bourdain has a drink at Tintol restaurant in Times Square. Bourdain, 49, is the star of 'Anthony Bourdain: No Reservations,' the Travel Channel series that's half travelogue and half food show. Traveling constantly for the show, on which he regularly grosses viewers out by eating such delicacies as fermented shark meat in Iceland - 'the smell alone would stop a rhinoceros in its tracks,' he says - means he's only in his Manhattan apartment a few days out of the month. (Photo by James Keivom/NY Daily News Archive via Getty Images)
NEW YORK - APRIL 21: Chef Tom Colicchio and TV personality Anthony Bourdain attend the Food Bank For New York City's Sixth Annual Can-Do Awards at Abigail Kirsch's Pier Sixty at Chelsea Piers on April 21, 2009 in New York City (Photo by Jamie McCarthy/WireImage for BWR)
MIAMI BEACH, FL - FEBRUARY 28: Eric Ripert and Anthony Bourdain attend the 2010 South Beach Wine and Food Festival Grand Tasting Village on February 28, 2010 in Miami Beach, Florida. (Photo by Alexander Tamargo/Getty Images)
MIAMI BEACH, FL - JUNE 30: Anthony Bourdain discusses his book 'Medium Raw: A Bloody Valentine to the World of Food and the People Who Cook' presented by Books and Books at Lincoln Theatre on June 30, 2010 in Miami Beach, Florida. (Photo by Gustavo Caballero/Getty Images)
TOP CHEF -- 'An Offer They Can't Refuse' Episode 808 -- Pictured: (l-r) Jduges Tom Colicchio, Anthony Bourdain (Photo by David Giesbrecht/NBC/NBCU Photo Bank via Getty Images)
Anthony Bourdain attends the Whole Foods Market Grand Tasting Village during the 2011 South Beach Wine and Food Festival on February 27, 2011 in Miami Beach, Florida. (Photo by John Parra/WireImage)
NEW YORK, NY - MAY 19: TV personality Anthony Bourdain attends the Great Googa Mooga 2012 at Prospect Park on May 19, 2012 in New York City. (Photo by Ilya S. Savenok/Getty Images)
LATE NIGHT WITH JIMMY FALLON -- Episode 650 -- Pictured: (l-r) Anthony Bourdain, Jimmy Fallon -- (Photo by: Ira James/NBC/NBCU Photo Bank via Getty Images)
THE TASTE - ABC's 'The Taste' features no-holds barred chef Anthony Bourdain. (Photo by Sasha Shemirani/ABC via Getty Images)
PASADENA, CA - JANUARY 10: Anthony Bourdain attends the Disney ABC Television Group 2013 TCA Winter Press Tour at The Langham Huntington Hotel and Spa on January 10, 2013 in Pasadena, California. (Photo by Jason LaVeris/FilmMagic)
MIAMI BEACH, FL - FEBRUARY 23: Anthony Bourdain attends the South Beach Wine and Food Festival 2013 Grand Tasting Village on February 23, 2013 in Miami Beach, Florida. (Photo by John Parra/Getty Images)
LAS VEGAS, NV - NOVEMBER 10: TV personality Anthony Bourdain attends 'Parts Unknown Last Bite' Live CNN Talk Show hosted by Anthony Bourdain at Atomic Liquors on November 10, 2013 in Las Vegas, Nevada. 24280_001_0026.JPG (Photo by Isaac Brekken/WireImage)
NEW YORK, NY - APRIL 10: (L-R) Senior vice president for talent and content development Amy Entelis, Anthony Bourdain and Senior vice president of development and acquisitions Vinnie Malhotra attend the CNN Upfront 2014 at Skylight Modern on April 10, 2014 in New York City. 24679_002_0417.JPG (Photo by Bryan Bedder/WireImage for Turner Networks)
NEW YORK, NY - MARCH 04: Chef, Author and TV Personality Anthony Bourdain attends The 2015 Bronx Academy Of Letters Chefs' Tasting Benefit at The Edison Ballroom on March 4, 2015 in New York City. (Photo by Ben Hider/Getty Images)
LOS ANGELES, CA - SEPTEMBER 12: Anthony Bourdain attends the 2015 Creative Arts Emmy Awards at Microsoft Theater on September 12, 2015 in Los Angeles, California. (Photo by Jason LaVeris/FilmMagic)
NEW YORK, NY - DECEMBER 08: Author/chef Anthony Bourdain attends the 2015 Center For Fiction Benefit & Awards at The Metropolitan Club on December 8, 2015 in New York City. (Photo by Mireya Acierto/Getty Images)
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To be fair, Bourdain retracted these notorious words in his more recent book Medium Raw, saying that this isn't necessarily the case for all NYC restaurants, anymore. But there are still certain places to be wary of like dive bars or chains where fish isn't the "main thrust of their business," Bourdain wrote in Medium Raw.
It all starts at the fish market
Bourdain spent most of his cooking career at different restaurants in New York City where most restaurant seafood is bought at the Fulton Street fish market in the Bronx — the second largest seafood market in the world.
During each weekday, buyers and sellers handle millions of pounds of seafood, which equates to over one billion dollars in daily sales.
But the market is closed on the weekend, and it's only open from 1 a.m. to 7 a.m. on Friday mornings. Chefs usually order the bulk of their fish on Thursdays in preparation for the busy weekend.
By the time Monday comes around, if the fish doesn't smell too fishy, then it might end up on your plate — depending on whether your chef actually cares about what he's doing — an attitude that is increasingly common in the cooking business nowadays, Bourdain wrote in Medium Raw.
Come Tuesday, the fish is likely too spoiled to sell and as the old fish is thrown out a new order comes in.
Your best days for ordering fish in NYC at a restaurant, then? Tuesdays and Thursdays, wrote Bourdain.