Work took place at Studio Three. Between 2.30 and 7.30pm they added a range of overdubs, not all of which were used in the final mix.
Paul McCartney recorded his lead vocals alone on one track, and on another he added a Rickenbacker bass part. The latter was taped along with falsetto backing vocals by John Lennon and George Harrison. Other overdubs included a piano part through a Leslie speaker played, and a Vox Continental part, both by George Martin, and extra lead guitar fills by Harrison.
A reduction mix allowed a fourth track to be cleared. Onto this were added more backing vocals by Lennon and Harrison, including the chorus and the famous 'Frère Jacques' countermelody.
Between 7.30 and 8pm two mono mixes of Paperback Writer were made in the control room of Studio Three. This featured heavy tape echo in the vocals of the chorus refrain, which was toned down somewhat on the later stereo mix.
Work then began on Rain, with five takes recorded before work ended at 1.30am.
One of the things we discovered when playing around with [tape] loops on Tomorrow Never Knows was that the texture and depth of certain instruments sounded really good when slowed down. With Rain The Beatles played the rhythm track really fast so that when the tape was played back at normal speed everything would be much slower, changing the texture. If we'd recorded it at normal speed and then had to slow the tape down whenever we wanted to hear a playback it would have been much more work.
The Complete Beatles Recording Sessions, Mark Lewisohn
The normal tape speed was 50 cycles per second, but the session it was recorded at 42 cycles. The backing track had drums and Lennon's distorted Gretsch Nashville guitar, and Lennon's lead vocals on another track. The fifth and final take was the only one to feature vocals.
It is likely that McCartney also recorded a bass part on this day, but it was replaced on 16 April during a session that saw a range of further overdubs, including backwards vocals in the coda.