"Why are people so tired on April…

A popular April riddle is:

Q: Why are people so tired on April 1st?
A: Because they just finished a 31-day March.

The joke—often involving soldiers or scouts—has been cited in print since at least 1920.

20 June 1920, Sunday World-Herald (Omaha, NE), Children’s Page, sec. 3, pg. 14, col. 2:
Why should soldiers always be tired on April 1, because they have just had a 31 day March.
(...)
IRENE FERNOW.

Newspapers.com
2 April 1927, Columbus (NE) Daily Telegram, “Scratches by the Office Cat,” pg. 4, col. 4:
No wonder we feel so tired the early part of April— we have just completed a 31-day’s March!

Newspapers.com
24 January 1947, The Spokesman-Review (Spokane, WA), “Horse Sense Quiz” by Dr. George W. Crane, pg. 4, col. 3:
On which day would the army infantry be most tired?
April 1 (following a 31-day march.)

Google Books
1968, Boys Life, “Think & Grin,” pg. 74, col. 1:
Harv: Why are Boy Scouts so tired on the first day of April?
Ted: Because they’ve just had a 31-day March.-George King, Jr., Cincinnati, Ohio.

Google Groups: rec.humor
Life 4.D
Henry Cate
5/6/93
(...)
Why were the Scouts so tired on April 1?
Because they had just finished a 31 day March.

Google Groups: rec.humor
Life 4.D
Henry Cate
10/12/95
(...)
Why were the Scouts so tired on April 1?
Because they had just finished a 31 day March.

Google Groups: alt.callahans
Puns of the Weak: 4-21-00
Stan Kegel
4/22/00
The army was very tired on April First because they had just completed a 31 day March. (Archives)

Twitter
Brad Boydston
@boydston
Our friend Laura Rodriguez in Mexico sent this groaner: Why are people so tired on April 1st?... They just finished a 31-day March.
5:52 PM · Mar 31, 2009·Twitter Web Client

Reddit—Jokes
Posted by u/Aemon_Targaryen April 2, 2016
Why are soldiers always so tired on April 1st?
Because they have just finished a 31 day March.

Twitter
Brent Housing
@brenthousing
Why are people so tired on April 1st?
It’s because they just finished a 31-day March
Don’t forget to play a healthy prank on someone today, it’s April Fool’s Day!
#brenthousing #AprilFools #Aprilfoolsday2020 #RealEstate #abujarealtor #AbujaLockdown #StayAtHome
6:20 AM · Apr 1, 2020 from Karu, Nigeria·Twitter for Android

“Why are people so tired on April 1st?”/“Because they just finished a 31-day March.”