What is "that'll be the day"

phr. (context idiomatic English) (non-gloss definition: Said in reply to something that one believes will never happen.)

"That'll Be the Day" is a song written by Buddy Holly and Jerry Allison. It was first recorded by Buddy Holly and the Three Tunes in 1956 and was re-recorded in 1957 by Holly and his new band, the Crickets. The 1957 recording achieved widespread success. Holly's producer, Norman Petty, was credited as a co-writer, although he did not contribute to the composition.

Cover versions have been recorded by various artists. It was the first song recorded (as a demonstration disc) by the Quarrymen, the skiffle group that evolved into the Beatles.

The 1957 recording was certified gold (for over a million US sales) by the Recording Industry Association of America (RIAA) in 1969. It was inducted into the Grammy Hall of Fame in 1998. It was placed in the National Recording Registry, a list of sound recordings that "are culturally, historically, or aesthetically important, and/or inform or reflect life in the United States", in 2005.

That'll Be the Day is a 1973 British drama film starring David Essex, Rosemary Leach and Ringo Starr, written by Ray Connolly and directed by Claude Whatham. It is set in the late 1950s/early 1960s and was partially filmed on the Isle of Wight.

That'll Be The Day is the second and final studio album from Buddy Holly. Decca, Holly’s first major record label, after failing to produce a hit single from Holly’s early recordings, packaged these 1956 tunes after he had some success with recordings from the Brunswick and Coral labels, i.e. the previously released single " That'll Be the Day". This is the last album released before his death in a plane crash on February 3, 1959, and is rare among collectors.

That'll Be the Day (abbreviated as TBTD) is a touring rock ’n’ roll show playing theatres and concert halls throughout the United Kingdom and Great Britain. That’ll Be The Day was first performed in 1986 and became the longest running 'one-night' touring show in the United Kingdom. The show, which currently comprises six vocalists and five musicians, is rewritten annually to incorporate new and used material. Since its creation, That’ll Be The Day has been written, directed and produced by Trevor Payne, who performs in the show.

The show reinvents itself on an annual basis, with a 27th year production touring throughout the United Kingdom during the summer and autumn of 2012 and into the spring of 2013.