Excerpted from 'John Lennon/Plastic Ono Band – The Ultimate Collection,' John and Yoko discuss the classic song.
The 1971 chart-topper was the latest product of the T. Rex team that helped create Bolanmania.
The set features 87 never-before-heard recordings, among 159 tracks across six CDs and two Blu-ray audio discs.
The February 7, 1970 issue of the magazine announced John as their Man of the Year.
On January 16, 1971, it was farewell Tyrannosaurus Rex, long live T. Rex the band and T. Rex the LP.
The live album was credited not to Lennon and Ono but solely to the Plastic Ono Band.
The UNICEF event featured John and George's first scheduled performance since The Beatles' last concert in 1966, and Lennon's last UK live appearance.
'Happy Xmas (War Is Over)' became a cast-iron Yuletide favourite, but its original UK peak was No.4.
The song was recorded during the couple's famous "bed-in" at the Queen Elizabeth Hotel in Montreal.
John Lennon/Plastic Ono Band, one of the most powerful statements of his entire career, made its debut on the UK chart on 16 January, 1971.