As you've probably observed, I'm completely John and Yoko obsessed. I know nothing about the context of this picture except it was probably in 1968 as John is in his Let It Be look (National Health Granny Glasses, clean shaven, with long hair). I can speculate though, it may have been for the Rolling Stone's Rock and Roll Circus. A post-psychedelic era, circus-themed rock show where the Stones shared the stage to play some recent hits with other rock artists of the day like John Lennon, Eric Clapton, and others. If you haven't seen it, its a great movie to get your mitts on. At the time it was completed, it could not be released due to some sort of legal battle behind the scenes. It was only recently finally made public, and you can purchase a copy on DVD.
In the above photo John looks like some sort of cross-dressing court jester and Yoko's costume is even funnier because she was constantly lampooned and berated as looking like a witch by the papers of the day-- And even though I am a huge Yoko fan and supporter I get the comparison: pale skin, long long black hair, tiny frame-- not to mention she clearly has a lovespell on Lennon. Its just like John and Yoko's self-depricating sense of humor though, to acknowledge the remarks and get in on the joke, so I can only imagine the witch costume was intentional.
I really love love dressing up for costume parties but living in Brazil right now, Halloween is not a celebrated holiday here so I will have to live vicariously through you. What are you going as?