“Win on Sunday, sell on Monday” is an auto racing adage. The races are usually held on Sunday; an automobile company that had a winning car on Sunday was said to have had increased sales on Monday. “Win Sunday and you sell Monday is the stock car racing code” was cited in Sports Illustrated on December 25, 1967. Ford Motor Company put the saying in a press release in 1968.
 
 
Sports Illustrated
December 25, 1967
A Journey Into Speed
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Win Sunday and you sell Monday is the stock car racing code, and in this big and bitter warfare—awash with loyal fans, swift pit crews and lightning engine changes—Ford’s rookie driver of the year, Donnie Allison (streaking into view below), is a key weapon against the Chrysler enemy.
 
17 January 1968, New York (NY) Times, “Ford and Plymouth To Renew Rivalry On Coast Sunday” by John S. Radosta, pg. 59:
WIN Sunday, sell Monday. That basic tenet of auto racing,
 
17 May 1969, Aberdeen (SD) American-News,  “Detroit Has Its Hand In The Big Business Of Racing” (AP), pg. 3, col. 1:
“Win on Sunday, sell on Monday,” said a Ford press release last year.
 
29 December 1969, Greensboro (NC) Record, “Two Blows For Safety” (editorial), pg. A18, col. 2:
“Win on Sunday, sell on Monday”, one Ford press release of some time ago declared.
 
Google Books
Wheels
By Arthur Hailey
Garden City, NY: Doubleday
1971
Pg. 382:
Most major races were on Sunday, and manufacturers of cars, tyres, and other equipment acknowledged the dictum: Win on Sunday, sell on Monday.
   
Motor Authority
Study: ‘Win On Sunday, Sell On Monday’ Still Holds True
By Nelson Ireson
Aug 4, 2010
A study released today has found that there’s a strong link between participation in motorsports and sales on dealer lots. Back in the mid-20th century, the association between winning races on Sunday and selling cars on Monday was an accepted fact. But over the past few decades, that link has been called into question—some suggesting it doesn’t even exist in today’s import-heavy marketplace.
 
The reasoning behind the lost link was that motorsports had become less relevant to today’s average road car, and buyers were looking for more than raw performance in a new vehicle purchase. But it turns out that while those facts may remain true, the benefit comes more from the people who attend the races than from the cars that participate in them.
 
Windsor (Ontario) Star
NASCAR gears up for MIS race
Bob Duff
Jun 11, 2013 - 7:42 PM EDT
Last Updated: Jun 11, 2013 - 9:23 PM EDT
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The state is home base for all of the major North American automakers, so doing well there is considered vital. There’s an age-old saying among manufacturers in auto racing – win on Sunday, sell on Monday – and those in the game will tell you it’s an absolute truth.