According to the Royal News blog:
Queen Elizabeth II, Prince Philip, Prince William, Princess Anne…attend a commemoration of the 800th anniversary of the signing of the Magna Carta on June 15, 2015 at Runnymede, a water-meadow near Windsor Castle.
England’s King John signed the Magna Carta, or Great Charter, at Runnymede on June 15, 1215 (depicted below…).
To read the entire post on the Royal News blog, please click here.