The band announced that they will play their 1994 breakout album Dookie in full during the secret set, in an interview on Radio 1 last night (August 22).
Billie Joe Armstrong first hinted at a surprise Reading Festival appearance earlier this month, tweeting: "Hello England! What rhymes with shredding??!!"
It will be Green Day's first performance at Reading since they headlined the event eight years ago in 2004.
Green Day also confirmed plans for a full headline tour in 2013 and will start the UK leg at London's Emirates Stadium on June 1.
The band will release Uno! as the first of three new records in the UK on September 24. Dos! follows in November, while Tré! will be released in January 2013.
Watch the music video for Green Day's 'When I Come Around' below: