“Don’t quit your day job” (or “Don’t give up your day job” or “Keep your day job”) is advice to an actor/artist/model/musician/writer who probably won’t be able to make a living in that field. “Don’t give up your day job” has been cited in print since at least 1931 and 1951.
 
 
(Oxford English Dictionary)   
colloquial. don’t give up the (also your) day job and variants: used to express humorous scepticism about another person’s (artistic) endeavour or talent, or to deter someone from a risky venture thought unlikely to succeed.
1951   Billboard 16 June 3/2   Berle, I caught you on TV. Don’t give up the day job.
1973   ‘Country Gazette’ (title of record)  Don’t give up your day job.
1982   N.Y. Times 16 May ii. 31   Mr. Perlman does fine, too, but he probably shouldn’t give up the day job just yet.
   
Newspapers.com
22 March 1931, Brooklyn (NY) Citizen, “Musical Angles” by David Gornston, pg. 10, col. 4:
... at least it will give you something to do ... and a room full of rejection slips is rather interesting ... that is, if you have a morbid sense of humor ... and by the way don’t give up your regular day job.
   
Newspapers.com
21 December 1965, Lincoln (NE) Evening Journal, “NU Coach Slays QB Club” by Curt Mosher, pg. 17, col. 4:
“You’re pretty funny but don’t ever give up your day job.”
 
Google News Archive
9 June 1973, Milwaukee (WI) Sentinel, “Honor for McGuire Follows a Roasting” by Rod French, pt. 1, pg. 20, col. 6:
To master of ceremonies Jaye, Mars suggested that comedy was not his field. “You’d better keep your day job, Aye Jaye,” Mars said.
 
OCLC WorldCat record
Don’t give up your day job
Author: Country Gazette (Musical group)
Publisher: Los Angeles : United Artists, 1973.
Edition/Format:  Music LP : Bluegrass music : English
 
25 February 1974, Seattle (WA) Daily Times, “Sonics rip 76ers in true ‘laugher’” by Bob Schwartzmann, pg. B1, col. 4:
A VOICE from the stands, and only he knew if it was directed at a player or a referee, advised:
 
“Don’t give up your day job.”
   
Google Books
The Last Laugh:
The world of the stand-up comics
By Phil Berger
New York, NY: Morrow
1975
Pg. 236:
In fact, one club owner advised, “Keep your day job, man.”
 
OCLC WorldCat record
The Michael Jackson special
Author: Steven R Dahlman; Don’t Quit Your Day Job (Comedy group)
Publisher: Des Moines, Iowa : Dahlman Creative Media, [1984]
Edition/Format:  Audiobook : Reel-to-reel tape : English
Summary: Presents radio advertising for the Michael Jackson special. Features the comedy group, Don’t Quit Your Day Job.
 
OCLC WorldCat record
Don’t give up your day job
Author: Jack Wagner
Publisher: Burbank, CA : Qwest Records, 1987.
Edition/Format:  Music LP : Rock music : English
 
OCLC WorldCat record
Some Advice on Writer’s Workshop: Don’t Quit Your Day Job.
Author: Jane Morelli Johnson
Edition/Format:  Article : English
Publication: Teaching and Learning Literature with Children and Young Adults, v7 n2 p56-61 Nov-Dec 1997
Database: ERIC The ERIC database is an initiative of the U.S. Department of Education.
Summary: Offers advice on surviving while trying to make a career as a writer after having been a writing teacher. Outlines the typical writing process students are expected to follow. Questions whether writer’s workshop demands the impossible of students. Finds that the best writing process is to write every day at a time that works for the particular individual. (PA)
   
OCLC WorldCat record
Don’t quit your day job! : adventures for the working stiff
Author: Jay H Toberman
Publisher: San Jose, Calif. : Writers Club Press, ©2000.
Edition/Format:  Book : English
 
OCLC WorldCat record
Don’t quit your day job : acclaimed authors and the day jobs they quit
Author: Sonny Brewer
Publisher: Douglas, Isle of Man : M P Pub., 2010.
Edition/Format:  Book : Biography : English
Summary: Collects anecdotes from contemporary American authors on the jobs and careers they held before becoming successful authors, including Steve Yarbrough, Connie May Fowler, Daniel Wallace, and Cassandra King.