“A grandmother pretends she doesn’t know who you are on Halloween” is a saying that has been printed on many images. “A grandparent pretends he doesn’t know who you are on Halloween” was written by American humorist Erma Bombeck (1927-1996) and printed in many newspapers in November 1974.
   
“A grandmother pretends she doesn’t know who you are on Halloween” was printed in an “In Memoriam” notice in The Times Recorder (Zanesville, OH) on March 17, 1976. “A grandparent pretends she doesn’t know who you are on Halloween” was a Bombeck quote printed in the Evansville (IN) Courier on November 17, 1976.
   
The popular version of the saying involves “grandmother” and not “grandparent.”
   
     
Wikipedia: Erma Bombeck   
Erma Louise Bombeck (née Fiste; February 21, 1927 – April 22, 1996) was an American humorist who achieved great popularity for her newspaper column that described suburban home life from the mid-1960s until the late 1990s. Bombeck also published 15 books, most of which became bestsellers. From 1965 to 1996, Erma Bombeck wrote over 4,000 newspaper columns, using broad and sometimes eloquent humor, chronicling the ordinary life of a midwestern suburban housewife. By the 1970s, her columns were read twice-weekly by 30 million readers of the 900 newspapers in the U.S. and Canada.
       
2 November 1974, The Republic (Columbus, IN), “At Wit’s End: Grandparent Defies Explanation” by Erma Bombeck (Publishers-Hall Syndicate), pg. 6, col. 4:
A grandparent pretends he doesn’t know who you are on Halloween.
     
Google Books
The Family Album
By Arthur DeMoss and Nancy DeMoss
Philadelphia, PA: A.J. Holman
1975
Pg. 52:
A grandparent pretends he doesn’t know who you are on Halloween.
   
Google Books
It’s a pleasure
By Marjorie Rawson
Poy Sippi, WI: Rawson
1975
Pg. 191:
Grandpa pretends he doesn’t know who you are on Halloween.
 
17 March 1976, The Times Recorder (Zanesville, OH), “In Memoriam,” pg. 6-B, col. 2 classified ad:
A grandmother pretends she doesn’t know who you are on Halloween.
 
17 November 1976, Evansville (IN) Courier, “‘Garbage hustler and pretty face’ torpedoes suburban life-style” by Scott Hill, pg. 19, col. 6:
(Erma Bombeck is the subject of this article.—ed.)
Grandparents—“A grandparent pretends she doesn’t know who you are on Halloween.”
 
Google Books
365 Reflections on Grandmothers
By Dahlia Porter and Gabriel Cervantes
Holbrook, MA: Adams Media Corporation
1997
Pg. 22:
A grandmother pretends she doesn’t know who you are on Halloween. —Erma Bombeck
   
Google Books
How Grand You Are
By Sarah Arak
Silverback Books Incorporated
2006
Pg. 23:
A GRANDMOTHER PRETENDS SHE DOESN’T KNOW WHO YOU ARE ON HALLOWEEN. ♢ -Erma Bombcck
     
Google Books
Women Know Everything!:
3,241 Quips, Quotes, & Brilliant Remarks
By Karen Weekes
Philadelphia, PA: Quirk Books; San Francisco, CA: Distributed in North America by Chronicle Books
2007
Pg. 201:
A grandmother pretends she doesn’t know who you are on Halloween. -ERMA BOMBECK (1927-1996)
 
Google Books
The Grandest Love:
Inspiring the Grandparent-Grandchild Connection
By Jerry Witkovsky
Xlibris LLC (Xlibris.com)
2013
Pg. 227:
“A grandmother pretends she doesn’t know who you are on Halloween.” —Erma Bombeck
 
Latin Times
15 Mother’s Day Quotes To Tell Grandma How Special She Is!
By Natalie Roterman | May 08 2016, 11:26AM EDT
(...)
“A grandmother pretends she doesn’t know who you are on Halloween.” – Erma Bombeck
 
Bustle
18 Grandma Mother’s Day 2019 Instagram Captions That Are Heartwarming & Unique
By BRITTANY BENNETT
May 7, 2019
(...)
“A grandmother pretends she doesn’t know who you are on Halloween.” - Erma Bombeck