This photo has been floating around on social media sites the past couple of days, prompting speculations about what the occasion was, until someone was able to sort it out. Turns out it's photo of Paul McCartney with Jorma Kaukonen & Paul Kantner from Jefferson Airplane in San Francisco, taken on 4th April 1967. McCartney is playing a right-handed Rickenbacker 360/12, which probably belonged to Paul Kantner.
McCartney and Mal Evans visited the Fillmore Auditorium, where Jefferson Airplane were rehearsing. Afterwards they accompanied Marty Balin and Jack Casady to the Oak Street apartment they shared with the band's road manager Bill Thompson. McCartney played them an acetate of the "Sgt Pepper's Lonely Hearts Club Band" album.
They also attempted a jam, but that was abandoned because all their equipment was right-handed, which McCartney had trouble playing.