Emperor Constantine I decreed that Sunday, the day honoring the sun god Sol Invictus (disc pictured), would be the Roman day of rest.

Constantine I, also known as Constantine the Great, was a 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 Rome, 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.

Sol Invictus was the official sun god of the late Roman Empire and a later aspect of, or replacement for, the old Latin god Sol. The emperor Aurelian revived his cult in 274 AD and promoted Sol Invictus as the chief god of the empire. From Aurelian onward, Sol Invictus often appeared on imperial coinage, usually shown wearing a sun crown and driving a horse-drawn chariot through the sky. His prominence lasted until the emperor Constantine I legalized Christianity and restricted paganism. The last known inscription referring to Sol Invictus dates to AD 387, although there were enough devotees in the fifth century that the Christian theologian Augustine found it necessary to preach against them.