One can see how that in the Roman Empire Sunday became a day of prestige and that among the mass of converted nominal Christians in the days of Constantine (and thereafter) it's prestige would hold sway over God's 7th day Sabbath.
"Sol Invictus ("Unconquered Sun") was the official sun god of the
later Roman Empire and a patron of soldiers.
On 25 December 274 AD the Roman emperor Aurelian made it an official cult alongside the traditional Roman cults....Where previously priests of Sol had been simply sacerdotes and tended to belong to lower ranks of Roman society, they were now pontifices and
members of the new college of pontifices instituted by Aurelian. Every pontifex of Sol was a member of the senatorial elite, indicating that the priesthood of Sol was now highly prestigious......Emperors portrayed Sol Invictus on their official coinage, with a wide range of legends, only a few of which incorporated the epithet invictus, such as the legend SOLI INVICTO COMITI, claiming the Unconquered Sun as a companion to the Emperor, used with particular frequency by Constantine.....Constantine decreed (March 7, 321) dies Solis—day of the Sun, "Sunday"—as the Roman day of rest (Codex Justinianus 3.12.2)."
Wikipedia
But the seventh day is the sabbath of the LORD thy God:
Exodus 20:10
"Sol Invictus ("Unconquered Sun") was the official sun god of the
later Roman Empire and a patron of soldiers.
On 25 December 274 AD the Roman emperor Aurelian made it an official cult alongside the traditional Roman cults....Where previously priests of Sol had been simply sacerdotes and tended to belong to lower ranks of Roman society, they were now pontifices and
members of the new college of pontifices instituted by Aurelian. Every pontifex of Sol was a member of the senatorial elite, indicating that the priesthood of Sol was now highly prestigious......Emperors portrayed Sol Invictus on their official coinage, with a wide range of legends, only a few of which incorporated the epithet invictus, such as the legend SOLI INVICTO COMITI, claiming the Unconquered Sun as a companion to the Emperor, used with particular frequency by Constantine.....Constantine decreed (March 7, 321) dies Solis—day of the Sun, "Sunday"—as the Roman day of rest (Codex Justinianus 3.12.2)."
Wikipedia
But the seventh day is the sabbath of the LORD thy God:
Exodus 20:10