From Legal Writing Prof Blog:
Although calendars have changed over time, the date on which King John signed Magna Carta was June 15, 1215. So today we can celebrate its 796th anniversary!
In the history of legal writing, Magna Carta is, as its name suggests, a document of great significance. It memorializes an absolute monarch's agreement to share power with his barons, arguably the first step in a democratizing process that's brought us all the way to this year's Arab Spring.
Magna Carta was written in medieval Latin, but you can read one of the three generally-accepted English translations here, on the British Library's website. That site also provides more context and explanations here.