The motto of the United States Navy SEALs is written on the Basic Underwater Demolition/SEAL (BUD/S) Training course at the Naval Special Warfare Training Center in Coronado, California:
 
“The Only Easy Day Was Yesterday.”
 
The motto means that every new day presents itself with a tough challenge; harder work is necessary. The SEALs motto was printed in a Parade magazine article in August 1987.
   
“The only easy day was yesterday”—infrequently given as “The only easy day is yesterday”—has been printed on many images and have been used in several book titles.
     
 
16 August 1987, Arizona Daily Star (Tucson, AZ), “Seals Are The Nay’s Elite Amphibious Force: Are They the Toughest Men Alive?” by David Fairbank White, Parade magazine, pg. 5, col. 1:
The base where sailors are transformed into SEALs is a sand-colored compound in Coronado, Calif., just down the bay from San Diego. It is indistinguishable from any other military base in the United States, but for two structures: a 50-foot tower used for diving exercises and an extensive obstacle course with barbed-wire tunnels and a 60-foot cargo net, among other traps, to scale. Deep in the central section of this base, dominating the main courtyard, stands a great big sign that reads: THE ONLY EASY DAY WAS YESTERDAY.
   
Los Angeles (CA) Times
The Gulf War Class : Latest SEAL Group to Graduate Is First Since Start of War
February 17, 1991 | NORA ZAMICHOW | TIMES STAFF WRITER
(...)
“If I knew then what I know now, it would be more intimidating because it doesn’t get any easier,” said Gunner’s Mate Kenneth Needham, 20. “They say the only easy day is yesterday, and it’s true.”
     
Google Books
SEAL!:
From Vietnam’s PHOENIX Program to Central America’s Drug Wars
By Michael J. Walsh with Greg Walker
New York, NY: Pocket Books
1994
Pg. 18:
During future missions SEALs will look back on BUD/S and remember that “the only easy day was yesterday,” and they’ll get on with accomplishing the mission.
 
OCLC WorldCat record
The only easy day was yesterday : an inside look at the training of the Navy SEALs
Author: Richard D Schoenberg
Publisher: Los Angeles, CA : Schoenberg & Associates, ©2004.
Edition/Format:   Print book : English : 1st American ed
 
OCLC WorldCat record
The only easy day was yesterday : fighting the war on terrorism
Author: A A Bufalo
Publisher: [Place of publication not identified] : S & B Publishing, ©2004.
Edition/Format:   Print book : English : Rev. ed
 
OCLC WorldCat record
SEAL warrior : the only easy day was yesterday
Author: Thomas H Keith; J Terry Riebling
Publisher: New York : St. Martin’s Griffin ; Godalming : Melia [distributor], 2010.
Edition/Format:   Print book : English
Database: WorldCat
Summary:
Offers an insight into the ways in which the SEALs fought, learned, reacted, and expanded their understanding of guerilla warfare during the Vietnam War.
 
OCLC WorldCat record
No easy day : the autobiography of a Navy SEAL : the firsthand account of the mission that killed Osama Bin Laden
Author: Mark Owen; Kevin Maurer
Publisher: New York, New York : Dutton, [2012]
Edition/Format:   Print book : Biography : English
Database: WorldCat
Summary:
For the first time anywhere, the first-person account of the planning and execution of the Bin Laden raid from a Navy Seal who confronted the terrorist mastermind and witnessed his final moments. Writing under a pseudonym to protect his anonymity, a member of the elite SEAL Team Six provides a behind-the-scenes, first-person account of all his dangerous assignments around the globe, including the training, planning
   
Military1
‘The only easy day was yesterday’
Motivate yourself to put hard times behind you — and then settle in for more rough road ahead
By Eric Davis, GovX January 17, 2014
We let too many powerful, life-changing quotes and sayings pass through our ears without taking any action on them. It’s time to take pause, listen and then actually change our lives because of them.
 
Let’s take this one.
 
“The only easy day was yesterday”
 
This famous saying is etched above the grinder in the BUD/S compound. Every bleeding back, bruised knuckle and searing muscle produced during SEAL physical training is underneath this sign
   
Chris Sajnog
The Top 10 Navy SEAL Sayings and Their Meanings — Motivational Quotes, Images and Definitions (SlideShare)
in NAVY SEAL TRAINING by Chris Sajnog | October 5, 2014
1. “The Only Easy Day Was Yesterday.”
Meaning: Every day you will need to work harder than the last. But when you work hard every day and see what you’re now capable of — yesterday seems easy.