Above, a 1934 plaque from the Big Apple Night Club at West 135th Street and Seventh Avenue in Harlem. Discarded as trash in 2006. Now a Popeye's fast food restaurant on Google Maps.

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Entry from May 16, 2015
“How many months have 28 days?” (riddle)

There is a popular riddle about the days in the month:

Q: Some months have 31 days, some months have 30 days. How many months have 28 days?
A. All twelve of them.

February, of course, is the only month that has 28 days and no more, but all the months have at least 28 days. The joke has been cited in print since at least 1896.

12 January 1896, The Daily News (Denver, CO), “Children’s Cute Sayings,” pg. 19, col. 7:
The teacher asking questions—teachers are quite apt to ask questions, and they sometimes receive curious answers. The question was as follows: “Now, pupils, how many months have 28 days?” “All of them, teacher,” replied the boy on the front seat.—Utica Observer.

Chronicling America
31 January 1896, Yorkville (SC) Enquirer, pg. 4, col. 2:
The teacher asking questions—teachers are quite apt to ask questions, and they sometimes receive curious answers. The question was as follows: “Now, pupils, how many months have 28 days?” “All of them, teacher,” replied the boy on the front seat.

6 November 1961, Pittsburgh (PA) Press, “Just For Fun” by Gilbert Love, pg. 21, cols. 1-2:
4—Some months have 30 days; some have 31. How many have 28?
(...)
4—This is awful. All the months have 28 days.

9 August 1965, The Free Lance-Star (Fredericksburg, VA), “Fun Time—The Riddle Box,” pg. 5, col. 2:
1. How many months have 28 days?

2 August 1970, Boca Raton (FL) News, “Mitch Woodbury Reports,” pg. 7A, cols. 1-3:
6. Some months have 31 days, some have 30 days. How many months have 28 days?
(...)
6. All months.

The Snark Puzzle Book
BY Martin Gardner
New York, NY: Simon and Schuster
1973
Pg. 35:
Some months have 30 days, some have 31 days. How many months have 28 days?

Pocket Smiles
By Robert C. Savage
Wheaton, IL: Tyndale House
1984
Pg. 36:
Some months have 30 days, some have 31 days. How many months have 28 days?