“Why is it getting harder to buy Advent calendars?”/“Because their days are numbered.”

An Advent calendar tracks the days between the first Sunday of Advent (a day from November 27 to December 3) until Christmas (december 25), A popular riddle is:
 
Q: Why is it getting harder to buy Advent calendars?
A: Because their days are numbered.
 
“Mm I need to buy an advent calender… and soon. My days are numbered, after all” was posted on Twitter on December 1, 2011. The joke went viral in 2013. “Why is it getting harder to buy Advent calendars? Because their days are numbered” was posted on Twitter on October 1, 2013.
 
“What happened to the man who stole an Advent calendar?”/“He got 25 days” is another Advent calendar riddle.
     
   
Wikipedia: Advent calendar
An Advent calendar is a special calendar used to count the days of Advent in anticipation of Christmas. Since the date of the First Sunday of Advent varies, falling between November 27 and December 3 inclusive, many Advent calendars, especially those that are resusable, often begin on December 1, although those that are produced for a specific year often include the last few days of November that are part of the liturgical season. The Advent calendar was first used by German Lutherans in the 19th and 20th centuries but is now ubiquitous among adherents of many Christian denominations.
     
Twitter
The Cethan Clause
‏@CethanLeahy
Mm I need to buy an advent calender… and soon. My days are numbered, after all.
9:03 AM - 1 Dec 2011
 
Twitter
Tony Cowards
‏@TonyCowards
Why is it getting harder to buy Advent calendars? Because their days are numbered. #GoldCrackers @GoldTVChannel
5:48 AM - 1 Oct 2013
 
Twitter
Glen Clarence
‏@glenclarence83
Just about to buy an advent calendar, not sure if it’s a good investment though, did a bit of research and it says there days are numbered!
8:02 AM - 22 Nov 2013
 
Twitter
Chris Heaton-Harris
‏@chhcalling
Why is it getting harder to buy Advent calendars?
Is it because their days are numbered?
2:02 AM - 29 Nov 2013
   
Mirror (UK)
New Christmas cracker jokes after modern makeover - what’s your favourite?
Groans at corny Christmas cracker jokes are as much of a festive tradition as the turkey - almost three-quarters of us believe the gags are outdated
BYRUKI SAYID
00:00, 10 DEC 2013 UPDATED11:12, 10 DEC 2013
(...)
8. Why is it getting harder to buy advent calendars?
Because their days are numbered.
 
Google Books
Gigglers: Christmas Jokes
By Toby Reynolds
London: Scholastic Fiction
2015
Pg. 89:
Q. Why is it hard to find advent calendar?
A. Because their days are numbered!
 
Google Books
Dick and Dom’s Christmas Jokes, Nuts and Stuffing!
By Dominic Wood and Richard McCourt
London: Pan Macmillan
2015
Pg. ?:
Why is it getting harder to buy Advent calendars?
Because their days are numbered.
 
Google Books
The Funny Christmas Stocking Filler Book
By Jonathan Swan
New York, NY: Random House
2016
Pg. ?:
Why is it getting harder to buy Advent calendars?
Because their days are numbered!
 
The Telegraph (UK)
50 best Christmas cracker jokes
By Jon Stock
1 DECEMBER 2016 • 9:30AM
(...)
Why is it getting harder to buy Advent calendars?
Because their days are numbered!