Emperor Constantine I decreed that Sunday, the day honoring the sun god Sol Invictus (disc pictured), would be the Roman day of rest. Constantine the Great: Constantine I, also known as Constantine the Great, was Roman emperor from AD 306 to 337 and the first Roman emperor to convert to Christianity. He played a pivotal role in elevating the status of Christianity in the Roman Empire, the Edict of Milan decriminalising Christian practice and ceasing Christian persecution. This was a turning point in the Christianisation of the Roman Empire. He founded the city of Constantinople and made it the capital of the Empire, which it remained for over a millennium. March 7, 321. Wiki link