Studio Two, EMI Studios, London
Originally planned for December 15th, (but there was no time to do it that day), "When I'm Sixty-Four" was adorned with the sound of three clarinets between 7:00 and 9:00 pm two ordinary and one a bass clarinet, played by Robert Burns, Henry MacKenzie and Frank Reidy. The recording was then mixed into mono, for acetate-cutting purposes, between 9:00 and 10:00 pm. Between 10:00 and session's end at 11:45 pm John's added yet more vocals and another piano track to take 26 of "Strawberry Fields Forever".
Source: The Complete Beatles Chronicle - Mark Lewisohn