Elton who was a fan of John's, had struck up a friendship with him. Elton had sang and played with John on the Whatever Gets You Through the Night single, which was culled from the Walls and Bridges album. He had also joined John for one other song on the album, Surprise Surprise (Sweet Bird of Paradox). John in turn played and sang on Elton's cover versions of John's songs, Lucy In The Sky With Diamonds and One Day (At A Time).
Elton had asked John to join him on stage, half-jokingly. John had not performed live for a couple of years, and was very reticent to join his friend on stage. But John agreed to do so provided his current single, Whatever Gets You Through the Night, went to number #1 on the charts. John said in an interview that he made this promise to Elton "never thinking in a million years [Whatever Gets You Through the Night] would go to #1." But of course, the single did reach #1 on the Billboard Charts, on November 16th, and John kept his promise.
John and Elton performed three songs- Whatever Gets You Through the Night, Lucy in the Sky With Diamonds, and I Saw Her Standing There, which John said was "written by an old fiancee of mine called Paul."
It was also at this concert that John and Yoko reunited after John's so-called "lost weekend". Yoko met John back stage after the show, and they ended their year and a half of estrangement.
Two years after the concert, Elton served as godfather for John and Yoko's son, Sean. And a mere six years later, John was murdered. Elton eulogized him in the song Empty Garden (Hey Hey Johnny).
This performance of two musical legends was a rare and memorable event in the history of rock and roll.
Below I have my Youtube play list of the performance, which is great. After Elton introduces John, they launch into a lively version of Whatever Gets You Through the Night, then Lucy in the Sky With Diamonds, and finally I Saw Her Standing There. I also included a second version of Whatever Gets You Through the Night. It has the actual audio from the concert and video from a movie called John & Yoko, a Love Story, with actors playing John, Elton and Yoko. It is well done and looks very real. Finally, there is a clip highlighting this momentous event from a 50th Anniversary program for Madison Square Garden.