“It is always darkest just before the Day dawneth” is a proverb from at least the 1600s. A modern jocular version, cited in print since at least 1996 and of unknown authorship, is:
 
“It’s always darkest just before the dawn. So if you’re going to steal the neighbor’s newspaper, that’s the time to do it!”
 
     
Google Books
I’m So Glad You Told Me What I Didn’t Wanna Hear
By Barbara Johnson
Dallas, TX: Word Pub.
1996
Pg. F83:
It’s always darkest just before the dawn. So if you’re going to steal the neighbor’s newspaper, that’s the time to do it!
 
8 June 2000, Augusta (GA) Chronicle, “Think before picking up that calculus book” by Glynn Moore, Richmond County Neighbors, pg. 2, col. 4:
It’s always darkest before dawn. So if you’re going to steal the neighbor’s newspaper, that’s the time to do it.
 
Google Books
Read This; It’s Funny!!:
A Laughable E-Mail Collection
By Carl E. Knox
Portland, OR: Our Computer Guy Incorporated
2003
Pg. 258:
It’s always darkest before dawn. So if you’re going to steal your neighbor’s newspaper, that’s the time to do it.
   
Adam’s Quotes
Saturday, April 16, 2011
Advice on When to Steal Your Neighbor’s Newspaper
It’s always darkest before the dawn. So if you’re going to steal your neighbor’s newspaper, that’s the time to do it.
—Author Unknown
   
Freakonomics
Is it Really Darkest Just Before the Dawn?
Fred Shapiro
08/04/2011 | 12:26 pm
(...)
It’s just not a reality that it’s always darkest just before the dawn.  According to The Yale Book of Quotations, the earliest known version of the saying is in Thomas Fuller‘s, A Pisgah Sight of Palestine (1650) (“It is always darkest just before the Day dawneth”), but that doesn’t help us with the puzzing question of why it arose.  Can any reader suggest an explanation?