Studio Two, EMI Studios, London
The mono mixes of "When I'm Sixty-Four" done on December 29th and marked "best" for the UK and US did not satisfy Paul. He suggested that they scrap all previous mixes and begin again, speeding up the new mix by as much as a semitone, a big difference. Such work began this 7:00 pm to 3:00 am session, and was followed by the making of simple tape-to-tape copies of the "best" mono mix of "Strawberry Fields Forever" from the previous afternoon. The remainder of the session was devoted to "Penny Lane", with a reduction of take six into take seven and an overdub of Paul's lead vocal, backed by John. It was well into the wee hours by the time these had been taped, so the other overdubs planned for the song had to wait until the next session and another year - 1967. All that was left to do in 1966 was mix into mono "Penny Lane", as it presently stood, so that an acetate disc could be cut.
Source: The Complete Beatles Chronicles