The Beatles had recorded the rhythm track on 8 April 1966. In the interim McCartney decided to add a brass section, made up of Eddie Thornton, Ian Hamer and Les Condon on trumpet, and Alan Branscombe and Peter Coe on tenor saxophone.
The Beatles wanted a definite jazz feel. Paul and George Martin were in charge. There was nothing written down but Paul sat at the piano and showed up what he wanted and we played with the rhythm track in our headphones. I remember that we tried it a few times to get the feel right, and then John Lennon, who was in the control room, suddenly rushed out, stuck his thumb aloft and shouted, 'Got it!' George Harrison got a little bit involved too but Ringo sat playing draughts in the corner.
The Complete Beatles Recording Sessions, Mark Lewisohn
With the overdub complete, three reduction mixes were made to create more space on the four-track tape. Previously-recorded vocals were judged to be no longer needed, so they were omitted, leaving only bass guitar, drums and brass. McCartney then added new vocals, accompanied by tambourine and organ. The fourth track was filled with a second lead vocal and an electric guitar part.
At the end of the session two mono mixes were created. The song wasn't complete, however: George Harrison added a guitar solo onto the first vocal track on 17 June, necessitating more mono mixes afterwards.