The signing of the Magna Carta occurred at Runnymede, a meadow by the River Thames near Windsor, on June 15, 1215. This document was a pivotal moment in English history, as it established the principle that the monarchy was not above the law. By agreeing to the Magna Carta, King John acknowledged certain rights of his barons, including protection from unlawful imprisonment and access to swift justice. It laid the foundation for constitutional principles and influenced the development of modern democratic systems, symbolizing a move towards greater legal rights for individuals and limiting the powers of the government.