15 June 1215 –
[from Wikipedia] John met the rebel leaders (Barons) at Runnymede, a water-meadow on the south bank of the River Thames, on 10 June 1215. Runnymede was a traditional place for assemblies, but it was also located on neutral ground between the royal fortress of Windsor Castle and the rebel base at Staines, and offered both sides the security of a rendezvous where they were unlikely to find themselves at a military disadvantage.
Here the rebels presented John with their draft demands for reform, the “Articles of the Barons”. Stephen Langton’s pragmatic efforts at mediation over the next ten days turned these incomplete demands into a charter capturing the proposed peace agreement; a few years later, this agreement was renamed Magna Carta, meaning “Great Charter”. By 15 June, general agreement had been made on a text, and on 19 June, the rebels renewed their oaths of loyalty to John and copies of the charter were formally issued.