“An optimist is a person who starts a new diet on Thanksgiving Day”

Chicago (IL) Sun-Times columnist Irv Kupcinet (1912-2003) is often credited with this seasonal quotation, from Kup’s Column:
 
“An optimist is a person who starts a new diet on Thanksgiving Day.”
 
It’s not known when the column first appeared in the Sun-Times, but it has been cited in print in other newspapers since November 1978.
 
   
Wikipedia: Irv Kupcinet
Irv Kupcinet (July 31, 1912 – November 10, 2003) was an American newspaper columnist for the Chicago Sun-Times, television talk-show host, and a radio broadcast personality based in Chicago, Illinois. He was popularly known by the nickname “Kup”.
 
His daily Kup’s Column was launched in 1943 and remained a fixture in the Sun-Times for the next six decades.
       
8 November 1978, The Telegraph (Folsom, CA), “The Stretch Stitch” by Bill Durban (ad. for Folsom Sew-Vac), pg. 2, col. 5:
An optimist is a person who starts a new diet on Thanksgiving Day.
 
Google News Archive
19 April 1979, The Spokesman-Review (Spokane, WA), “Shop with Sue” (ad.), pg. 5, col. 5:
An optimist is a person who starts a new diet on Thanksgiving Day.
 
Google Books
Quotable Quotes
By Editors of Reader’s Digest
New York, NY: Readers Digest
1997
Pg. ?:
An optimist is a person who starts a new diet on Thanksgiving Day.
—IRV KUPCINET, in Kup’s column
 
27 November 1997, Aberdeen (SD) American News, “Bridge” by Frank Stewart, pg. 7B, col. 5:
One definition of an optimist is somebody who starts a new diet on Thanksgiving Day.
 
Google Books
The Book of Thanksgiving:
Stories, Poems, and Recipes for Sharing One of America’s Greatet Holidays
By Jessica Faust and Jacky Sach
New York, NY: Citadel Press
2002
Pg. 31:
An optimist is a person who starts a new diet on Thanksgiving Day. — Irv Kupcinet
 
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“An optimist is a person who starts a new diet on Thanksgiving Day.”  Irv Kupcinet Give thanks to your #salesteam @MaxKudos #Thanksgiving
6:56 PM - 26 Nov 2015