“Why did the hotel hire a skeleton on Halloween?”/“He had a skeleton key.”

A skeleton key is a master key that opens many doors. “Now I know how ghosts get into haunted houses. They use skeleton keys!” was printed in the Washington (DC) Post on November 28, 1937. A Halloween riddle was printed in many newspapers in 1959:
 
Q: What kind of key would you use to open a graveyard?
A: A skeleton key.
 
A haunted house version of the joke was printed in a 1976 newspaper:
 
MOTHER: What would you like for your birthday?
GIRL: A skeleton key.
MOTHER: Why do you want that?
GIRL: To get into the haunted house!
 
A hotel version was printed in a 2016 book:
 
“A hotel hired a skeleton on Halloween night to open some hotel room doors, he had a skeleton key.”
 
“What kind of key opens a haunted house?”/“A spoo-key” is a similar riddle.
 
   
Wikipedia: Skeleton key
A skeleton key (also known as a passkey) is a type of master key in which the serrated edge has been filed down so that it can open numerous locks, most commonly the warded lock. The term derives from the fact that the key has been reduced to its essential parts.
 
In a broader sense the term can be used synonymously with master key to refer to any key, keycard or other device capable of opening a variety of locks.
       
28 November 1937, Washington (DC) Post, “Fun Corner,” Parade of Youth sec., pg. 8, col. 5:   
Superstitious Steve Says—     
“Now I know how ghosts get into haunted houses. They use skeleton keys!”
 
14 July 1959, Troy (NY) Record, “Fun Time—The Riddle Box,” pg. 3, col. 4:
What kind of key would you use to open a graveyard?
A skeleton key.
 
22 April 1966, Alton (IL) Evening Telegraph, “Fun Time—The Riddle Box,” pg. A-12, col. 5:
How can one get into a locked cemetery at night?
With a skeleton key?
 
6 November 1976, Hartford (CT) Courant, “Jokes and Riddles,” Lucky Day sec., pg. 4, col. 1:
MOTHER: What would you like for your birthday?
GIRL: A skeleton key.
MOTHER: Why do you want that?
GIRL: To get into the haunted house!
Ronnie Owens, 11, Purchase, New York
 
16 October 2000, Detroit (MI) Free Press, “Yak Yuks,” pg. 2: 
How do you get into a haunted house?
With a skeleton key.
 
18 October 2001, St. Louis (MO) Post-Dispatch, “Get Out,” pg. 19, col. 3:
How did the ghost get through the locked door?
He used a skeleton key!
 
Google Groups: rec.humor.jewish
Puns of the Weak 05/10/02
Stan Kegel
5/11/02
(...)
How do you unlock a haunted house?
With a skeleton key. (Daily Groaner)
   
Google Groups: alt.fifty-plus.friends
Have a HAPPY HALLOWEEN !!!
Tommy
11/1/03
(...)
Q: How can you enter a haunted house?
A: With a skeleton key.
 
Google Books
Puns Spooken Here:
Word Play for Halloween
By Richard Lederer
Illustratiosn by Jim McLean
Layton, UT: Wyrick & Company
2006
Pg. 38:
How do you unlock a haunted house?
With a skeleton key.
 
Straight Dope Message Board
Am I missing something with this Laffy Taffy joke?
FairyChatMom
09-30-2007, 07:02 AM
(...)
I wonder if they got the old classic: How do you unlock a haunted house? With a skeleton key!!
 
Google Books
Kooky Halloween Jokes to Tickle Your Funny Bone
By Linda Bozzo
Berkeley Heights, NJ: Enslow Publishers, Inc.
2013
Pg. 29:
What do you use to unlock the door of a haunted house?
A skeleton key.
 
Google Books
Happy Halloween Jokes 5
By Peter Crumpton
PeteyRF Creative
2015
Pg. ?:
What kind of key does a skeleton use?
A skeleton key.
 
Google Books
Jokes 101
By Alex Gall
Bloomington, IN: Balboa Press
2016
Pg. ?:
A hotel hired a skeleton on Halloween night to open some hotel room doors, he had a skeleton key.