A Christmas riddle is:
         
Q: What do you get when you cross an apple with a Christmas tree?
A: A pineapple.
 
The joke was posted on the newsgroup alt.humor.puns on December 27, 2002.
 
“What do you get when you combine a Christmas tree with an iPad?”/“A pineapple” is a related riddle.
 
 
Wikipedia: Christmas tree
A Christmas tree is a decorated tree, usually an evergreen conifer such as spruce, pine, or fir or an artificial tree of similar appearance, associated with the celebration of Christmas. The modern Christmas tree was developed in early modern Germany (where it is today called Weihnachtsbaum or Christbaum),
 
Google Groups: alt.humor.puns
Puns of the Weak 12/27/02
Stan Kegel
12/27/02
(...)
What do you get if you cross an apple with a Christmas tree ?
A pineapple! (Kids Jokes)
 
25 December 2003, Wisconsin State Journal (Madison, WI), “Dopey Jokes o’ the (Holi)Day,” pg. F1:
Q. What do you get if you cross an apple with a Christmas tree?
A. A pineapple.
   
Google Groups: alt.humor.puns
Kids Puns of the Weak 12-28-04
Stan Kegel
12/27/04
(...)
What do you get if you cross an apple with a Christmas tree?
A pineapple! (Daily Groaner)
 
Twitter
Frank Swift‏
@geeksjourney
Replying to @Morticia626
@Morticia626 What do you get when you cross an apple and a Christmas tree?  A pineapple!  *guffaws* *slaps knee*
1:18 PM - 18 Dec 2008
     
Google Books
Christmas Tree
By Louie Jerome
Lulu Press (Lulu.com)
2009
Pg. ?:
What do you get if you cross an apple with a Christmas Tree?
A pineapple.
 
Twitter
Seasonal World Pools‏
@seasonalworld
What do you get when you cross an apple and a Christmas tree? A Pineapple
3:14 PM - 7 Dec 2009
 
Google Books
Christmas Crackers
By Matt Wilce
Lulu Press (Lulu.com)
2012
Pg. ?:
What do you get if you cross an apple with a Christmas tree?
A pineapple!
   
Twitter
Ellen DeGeneres‏
@TheEllenShow
What do you get when you cross an apple with a Christmas tree? A Pineapple. #ChristmasJokeTuesday
2:39 PM - 25 Dec 2012
 
My Town Tutors
Parent Jokes: Top 50 Christmas Jokes for Parents
Posted on December 1, 2013 by Cassie Herd (Midwest City, OK)
(...)
48. What do you get if you cross an apple and a Christmas tree?… Pineapple.
   
12 December 2015, Daily News (Warwick, Queensland), “Joke of the Day,” pg. 2:
Q: What do you get when you cross a Christmas tree with an apple?
A: A pineapple.
     
Google Books
Christmas Jokes
By Delphon Coker
Xlibris Corporation (Xlibris.com)
2016
Pg. ?:
Joke 120: How can we get Pineapple?
Answer: By crossing a Christmas Tree with an Apple
 
Google Books
Happy Smile:
Jokes for Kids
By Adam Smith
Lulu Press (Lulu.com)
2017
Pg. ?:
Q: What do you get when you cross a Christmas tree with an apple?
A: A pineapple!