As to the Anthology BOOK Paul is equally vague, with no dates mentioned.
"I started thinking about what would be a really mad name to call a band. ... I took an idea back to the guys in London: 'As we're trying to get away from ourselves --- to get away from touring and into a more surreal thing --- how about if we become an alter-ego band, something like, say, "Sgt. Pepper's Lonely Hearts"? I've got a little bit of a song cooking with that title.' "
*This confirms some of herecomesthesun's ideas.
"'Sgt. Pepper' is Paul, after a trip to America.... He was trying to put some distance between The Beatles and the public --- and so there was this identity of Sgt. Pepper" - John Lennon, Anthology.
*Doesn't really say this was meant to be a concept album sort of reason, just a vague and 'trippy' thing to maybe play around with. IMO.
George Martin, Anthology: "Basically it was Paul's idea: he came in and said he had the song 'Sgt. Pepper's Lonely Hearts Club Band' and that he was identifying it with the band, with the Beatles themselves. We recorded the song first, and *then* (emphasis mine) the thought came to make it an idea for the album.
*In other words, Pepper was recorded (in 1967) and THEN the idea of the alter-ego came into play. In 1966 all Paul did was think of the trippy name. And yes, When I'm 64 was recorded in December 1966. But not for any concept except perhaps in keeping with the childhood theme that was *initially* the concept for Pepper (SFF and PL).
Again, going to the source:
In Many Years From Now (Miles/McCartney) it does confirm the Kenya/Nairobi flight (November 1966) being the one Paul came up with the idea, so obviously this is another example of imperfect memories contradicting themselves.
"Sgt. Pepper is often described as the first concept album, but it was not initially conceived as such. There was never an intention to make a themed album, a 'northern' album, or present a mini-opera as the Who did later. ... It freed them [the Pepper alter-ego idea] from their public image and allowed them to take a new, unfettered direction; it gave them the distance necessary to attempt something as extraordinary as 'A Day In The Life'.
Only later in the recording did Neil Aspinall have the idea of repeating the 'Sgt. Pepper' song as a reprise, and The Beatles and George Martin begin to use the linking tracks and segues to pull it all together, making it into more of a concept album."
*So this backs my contention that the concept (beyond the initial name) was come up with during the recording process and that that occurred in 1967. It is too bad the dates and such are not more precise on these events.
Again, it really is a case of both proponents being somewhat right and being somewhat wrong.