“Start every day with a smile” became a common saying by the 1930s. “Start every day with a smile and get it over with” is a jocular one-line saying that has been printed on many images. An article in the Los Angeles (CA) Times on October 22, 1967 credited “Start The Day WIth A Smile (And Get It Over With)” to Emil Reisman, a prolific writer of bumper sticker slogans.
 
The line has frequently been credited since the 1980s to American actor and comedian W. C. Fields (1880-1946), but there’s no evidence that he ever said it.
 
     
25 September 1933, Washington (DC) Post, pg. 10, col. 1:
Read the Washington Post’s Comics to Start Every Day With a Smile
 
Old Fulton NY Post Cards
5 June 1935, Daily Sentinel (Rome, NY), pg. 12, col. 7 ad:
Wouldn’t You Like to Start Every Day with a Smile?
There is a saying that runs like this: “If you can smile until ten in the morning the rest of the day will take care of itself.”
(Clark Laying mattresses.—ed.)
 
22 October 1967, Los Angeles (CA) Times, “west/view: One-Line Laureate of La Cienega” by Burt Prelutsky, West magazine, pg. 6, col. 2:
Other Reismanisms: (...) Start The Day WIth A Smile (And Get It Over With); ...
(Emil Reisman. This article is also on the Google News Archive.—ed.)
 
4 September 1969, Tucson (AZ) Daily Citizen, “Kids Back In School Yield A Few yarns” by Don Schellie, pg. 21, col. 1:
Like this sign, which we understand is stickered to the rear of a fancy Mercedes that courses our Tucson streets:
 
START THE DAY WITH A SMILE (and get it over with) ...
   
Google Books
The Comic Encyclopedia:
A Library of the Literature and History of Humor Containing Thousands of Gags, Sayings, and Stories
By Evan Esar
Garden City, NY: Doubleday
1978
Pg. 709:
One tired waitress to another: “I always start the day with a smile — and get it over with.”
 
2 December 1985, The Times (Trenton, NJ), “Watching the baby boomers” by Frank Tyger, pg. A2, col. 1:
“Start every day with a smile,” advised W. C. Fields, “and get it over with.”
 
Google Books
The 2,548 Best Things Anybody Ever Said
Edited by Robert Byrne
New York, NY: Fireside
2003
Pg. ?:
1,204
Start every day off with a smile and get it over with.
—W. C. Fields (1889-1946)
 
Google Books
Oxford Dictionary of Humorous Quotations
By Gyles Brandreth
New York, NY: Oxford University Press
2013
Pg. 8:
Start every day with a smile and get it over with.
W. C. Fields 1880–1946 American humorist;
attributed, perhaps apocryphal
 
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11:00 AM - 24 May 2016