“That’ll be the day!” (Buddy Holly; John Wayne; Kellogg’s cereal)

“That’ll Be the Day” was a 1957 hit song for Buddy Holly (1937-1959) of Lubbock, Texas. The inspiration for the song was the line “That’ll be the day!” spoken several times by John Wayne in the John Ford-directed western classic, The Searchers (1956).
 
The phrase “That’ll be the day!” is found in the newspaper comic “Joe Jinks” in 1938. “That’ll be the day!” appeared in Kellogg’s breakfast cereal ads in 1942, and then the phrase appears to have had a general circulation.
 
     
Yale Book of Quotations
edited by Fred R. Shapiro
New Haven, CT: Yale University Press
2006
Pg. 364:
Buddy Holly (Charles Hardin Holley)
U.S. rock singer and musician, 1937-1959
“That’ll be the day when I die.”
“That’ll Be the Day” (song)(1957). Cowritten with Jerry Allison and Norman Petty.
 
Wikipedia: Buddy Holly
Charles Hardin Holley (September 7, 1936 – February 3, 1959), better known as Buddy Holly, was an American singer, songwriter, and a pioneer of Rock and Roll. The change of spelling of “Holley” to “Holly” came about because of an error in a contract he was asked to sign, listing him as Buddy Holly. That spelling was then adopted for his professional career. The original spelling of “Holley” was engraved on his headstone (see photo).
(...)
Back in Lubbock, Holly formed his own band, The Crickets and began making records at Norman Petty’s studios in New Mexico, New Mexico. Among the songs they recorded was what became the hit version of “That’ll Be the Day”. Norman had music industry contacts and believing that “That’ll Be the Day” would be a hit single, he contacted publishers and labels. Coral Records, a subsidiary of Decca, signed Buddy Holly and The Crickets. This put Buddy in the unusual position of having two record contracts at the same time. Before “That’ll Be The Day” had its nationwide release, Holly played lead guitar on the hit-single “Starlight”, recorded in April, 1957, featuring Jack Huddle. The initial, unsuccessful version of “That’ll Be The Day” played more slowly and about half an octave higher than the hit version.
   
Internet Movie Database
Memorable Quotes from
The Searchers (1956)
Reverend Clayton: You wanna quit, Ethan?
Ethan: That’ll be the day.
(...)
Martin: I hope you die!
Ethan: That’ll be the day.
     
30 November 1938, Nebraska State Journal (Lincoln, NE), “Joe Jinks” comic, pg. 14:
“YOU? HA-HA-HA- THAT’LL BE THE DAY!”
YES, MIKE—THAT’LL BE THE DAY
AND IT ISN’T FAR OFF!!

2 September 1942, Winnipeg Free Press, pg. 4 ad:
And soon she’ll be old enough to dig into a breakfast bowl of Kellogg’s Corn Flakes. That’ll be the day!
     
21 February 1943, Lincoln (NE) Sunday Star and Journal, pg. 3, col. 1:
The 919th Squadron council is deep in session arranging plans for a squadron party to be held in the very near future. Better rush the plans, fellows. They may ration the main course. “That’ll be the day.”
 
17 September 1944, Valley Morning Star (Harlingen, TX), pg. 9, col. 2:
That’ll be the day.
 
29 December 1945, Port Arthur (TX) News, “Man About Town” by Walter Winchell, pg. 6:
That’ll be the day!