“When black cats prowl and pumpkins gleam, may luck be yours on Halloween” is a saying that has been printed on many images. “When black cats prowl and pumpkins gleam, May luck be yours on Hallowe’en” was printed on a special seasonal postcard that was mailed on October 25, 1912.
     
 
8 November 1930, Beaumont (TX) Journal, “Poet’s Corner,” The Junior Journal, pg. 3, col. 5:
When black cats prowl at night and pumpkins gleam,
Come, celebrate the approach of Halloween.
 
Google Books
Halloween:
Romantic Art and Customs of Yesteryear
By Diane C. Arkins
Gretna, LA: Pelican Publishing Company
2007
Pg. 26:
Hallowe’en Greetings
When black cats prowl and pumpkins gleam, May luck be yours on Hallowe’en.
 
“Mirror Series” postcard No. 248A, marked LSC. Mailed Gloucester, Massachusetts, October 25, 1912.
 
Twitter     
Stephen Weber
@stephenweber1
→_→ When black cats prowl and pumpkins gleam, May luck be yours on Halloween. ~Author Unknown~ ( http://tinyurl.com/6kzv4e )
11:40 AM - 28 Oct 2008
 
Google Books
Toasts:
Over 1,500 of the Best Toasts, Sentiments, Blessings, and Graces
By Paul Dickson
New York, NY: Bloomsbury USA
2009
Pg. 102:
When black cats prowl and pumpkins gleam, May luck be yours on Halloween. —Postcard greeting, circa 1920
   
ValueWalk
Halloween Quotes – BOO!
October 30, 2015 at 12:57 pm by Sheeraz Raza
(...)
When black cats prowl and pumpkins gleam, May luck be yours on Halloween.
 
Elite Daily
The 34 Best Instagram Captions For Halloween Are Here, So Spook On
BY MARISA CASCIANO
October 17, 2018
(...)
6. “When black cats prowl and pumpkins gleam, may luck be yours on Halloween.” — Unknown