Short answers, pulled from the story.
Constantine the Great saw a cross-shaped trophy formed from light above the sun at midday sometime between his father Constantius Chlorus's death in 306 and the Battle of the Milvian Bridge on the 28th of October 312. This event is recorded by Eusebius of Caesarea as occurring before the battle.
The Edict of Milan announced that it was proper for all people to follow the mode of religion which appeared best to them while returning confiscated Church property. This edict made the empire officially neutral regarding religious worship without making traditional religions illegal or Christianity the state religion.
On the 7th of March 321, Sunday became an official day of rest when markets were banned and public offices closed except for freeing slaves. This decree established Sunday as a legal day of rest throughout the empire under Constantine the Great.
Constantine the Great summoned the First Council of Nicaea effectively the first ecumenical council unless the Council of Jerusalem is so classified. The Council dealt primarily with the Arian controversy regarding who and what God was.
Just before his death in May 337, it is claimed that Constantine the Great was baptised into Christianity after being a catechumen for most of his adult life. He believed waiting until deathbed reduced danger of polluting soul with sin.