“Happy fall, y’all” (or “It’s fall, y’all”) is a seasonal saying that has been printed on many images. “IT’S FALL, Y’ALL” was printed in the Indianapolis (IN) News on September 23, 1966. “Happy fall y’all” was printed on the newsgroup alt.surfing on October 8, 1996.
 
   
23 September 1966, Indianapolis (IN) News, weather on left masthead, pg. 1:
IT’S FALL, Y’ALL,
 
19 September 1980, Omaha (NE) World-Herald, pg. 12, col. 5 ad:
IT’S FALL, Y’ALL.
TIME TO LANDSCAPE.
(Lanoha Nursery.—ed.)
 
22 September 1996, Fort Worth (TX) Star-Telegram, “Weeklong 1992 temblor in California lacked a whole lot of quakin’ going on,” pg. 10:
It’s fall, y’all. Happy fall to you.
     
Google Groups: alt.surfing
The Rincon Paradox
Surfer Bob
10/8/96
(...)
The west swells are here!
Happy fall y’all,
Surfer Bob
 
24 September 1997, Palladium-Item (Richmond, IN), Hot Ticket sec., pg. 3, col. 5 ad:
HAPPY FALL Y’ALL!
(Past & Presents.—ed.)
 
Google Books
October 2000, Cincinnati (OH) magazine, pg. 104, col. 1 ad:     
9/16-10/31
It’s Fall Y’all
Pumpkin Patch and Corn Maze
Henderson KY • 1-800-648-3128
 
Google Books 
Puttin’ on the Grits:
A Guide to Southern Entertaining
By Deborah Ford
New York, NY: Dutton
2005
Pg. 140:
Friends always look forward to our Happy Fall, Y’all party.
 
Twitter
hotash
@hotash
off to arrange some rocks, something round and some flowers - bbl / happy fall y’all
4:48 PM - 10 Sep 2008
     
Twitter
Reeder
@thatreeder
Eating first pumpkin scone of the year. It’s fall y’all. Just awesome.
11:20 AM - 19 Sep 2008
   
Twitter
Mel Tuttle
@meltuttle
I wore jeans today!  It’s fall y’all.
7:21 PM - 19 Sep 2008
 
Twitter
MommaB
@cherishbound176
Wooo—-Monday is already here?  Happy Fall y’all!  I LOVE this time of year!!  Up early and time to get some work done before kid invasion.
6:18 AM - 22 Sep 2008
   
Elite Daily
32 Quotes For First Day Of Fall If You’re All About “Pumpkin Spice & Everything Nice”
BY TESSA HARVEY
SEPT 22 2017
(...)
17. “It’s fall, y’all.”—Unknown
 
Elite Daily
Best Instagram Captions For Autumn That Are Perfect For Your Seasonal Snaps
BY GABRIELLE LAFRANK
SEPT 27 2017
(...)
12. “Happy fall y’all.”—Unknown
     
Google Books
Title Happy Fall Y’All Journal Notebook: 100 Pages 6 X 9 Lined Writing Paper Diary Planner Owl Pumpkin Wagon
Author Rengaw Creations
Publisher CreateSpace Independent Publishing Platform, 2018
ISBN 1721171398, 9781721171392
Length 100 pages
 
Twitter
Kylie
@kyliemacarthur
We’re making peach cobbler bars and Ashlyn keeps yelling happy fall y’all 🎃🍁🍂
11:58 PM - 1 Sep 2018