On February 10 1967 which was the day the orchestra recorded their crescendo for the "A Day In The Life" session, The Beatles found that there was really nothing after the orchestra overdub on tape to end the song in a long and drawn-out way. It was suggested to have the group (and probably some friends) stand around a microphone and "hum" a note together trying to make it last at long as possible. This was tried and rejected. This recording of the "hum" version ending can be found on the alternate bootleg of the "Anthology" DVD director's cut bootleg.
On February 22 1967 at EMI Studio 2, another idea spawned out of the group whereby keyboards would be used to sustain notes as long as possible for the ending. It was decided that three pianos and a harmonium would be used. Therefore, the ending for "A Day In The Life" consists of John Lennon, Paul McCartney, Ringo Starr, Mal Evans and (on harmonium) George Martin all play an E major chord together on the various pianos, etc.
It took nine attempts at this ending and the ninth was used on the recording. The final chord is almost a minute long (53 seconds to be exact according to John Winn's book). The sound was layered three times to thicken it up a bit and this was added to the mono and stereo mixes of the song completed the same day.