Paul McCartney paid a heartfelt tribute to John Lennon on the anniversary of his death, revealing it was a "sad, sad day".MORE: John Lennon and Paul McCartney's sons just took a selfie together and they look exactly like their famous fathersThe singer, 78, shared a throwback picture of them together, taken by Paul's former wife Linda McCartney, and wrote alongside it: "A sad sad day but remembering my friend John with the great joy he brought to the world."I will always be proud and happy to