Constantine the Great was the 57th Roman emperor. He finalised the termination of the persecution of Christianity, convened the Council of Nicaea to try to arbitrate disputes between rival Christian doctrines, built important Christian Churches, turned Byzantium into his imperial capital and renamed it Constantinople (City of Constantine) and issued new god coins.
Byzantium
Constantinapole was established by Consantine.
Consantine the great had proohetic dreams about the Christian god helping him win battle. This led Constantine to become the first Roman emperor to convert to Christianity.
Constantine the Great stabilised the Roman Empire by winning two civil wars (one against Maxentius, a usurper emperor in Italy, and one against his co-emperor, Licinius) and becoming sole emperor. The emperor Diocletian had set up the tetrarchy (rule by four) whereby the Roman Empire was ruled by two senior emperors (Augusti, plural of Augustus) and two junior emperors (Caesars), with each one of the four emperors in charge of a part of the empire. When Diocletian and the other senior emperor (Maximian) abdicated there were power struggles in the empire. When Constantius Chlorus (Constantine's father) who had been a Caesar and then became an Augustus, died a year later Constantine wanted the become the Augustus of the western part of the Roman Empire. However, he was appointed as only a Caesar. Meanwhile, Maxentius usurped the imperial title in Italy and made himself emperor of Italy and Africa. Constantine, in his bid to become an Augustus, organised political support in the area of the empire he controlled (Britain and Gaul) and then fought against Maxentius and defeated him. With this he became the emperor of the western part of the empire, while Licinius was the emperor of the eastern part. Constantine and Licinius then fought each other. Constantine won this civil war, too, and became the sole emperor of the whole empire. Having a sole emperor brought the civil wars to an end and stabilised the empire.
During the reign of Constantine I, Christianity was legalized, and became the official state religion. Constantine himself was Christian. The Edict of Milan legalized Christian practices during his reign, and later he established great Christian cities, such as Constantinople, which is today Istanbul.
It depends on your denomination. The Seventh Day Adventists worship God on Saturday. Some Christians do nothing on Saturday to keep the day holy and others do some stuff and worship God on Sunday the first day of the week. Consantine started the Sunday Worship. There are a lot of pros and cons for the day of the week to worship God. My answer is that everyday is a day to worship God and it does not hurt to Worship as a fellowship on the Sabbath as shown in the ten commandments. Although my denomination does not do that.