It isn't on Sunday. Jews still practice the Sabbath law. Christians worship on Sunday, the first day of the week. The day on which according to the Bible Jesus rose from the dead. The day early Christians established as their day of worship. C hristians generally do not call Sunday the Sabbath.
Classically, time was considered cyclical. Setting up a periodic ritual sets up a resonance in time. Making it one day in seven exploits the properties of 7. So there is a certain logic to the Sabbath. Observing the Sabbath connects you to 3000 years of Jews observing the Sabbath. Not a bad deal.
Christians did not build upon this resonance by joining in. They made their own separate ritual, which is what a cult would do.
It was partly influenced by the Mithras worshippers, which were very numerous in the early days of Christianity. Mithras was the god of the sun, and Sun-day was his day. (Mithraism also contributed more to Christianity: the birth on Dec.25, baptism in blood, birth witnessed by shepherds, etc.) The Romans also called it "the Lord's day," and it was observed as special. Early Chiristians observed both days, Sunday and Saturday (Sabbath), depending on whether they were Jews or Gentiles.
Some Christian sects today (including some Mormon offshoots) observe a Saturday Sabbath, besides the Seventh Day Adventists.
My question has always been: how do you know which day is the seventh day, when God wants you to rest? I have asked devout Adventists this question, and the best they can come up with is that the calendar is "very, very old." That doesn't mean that it's also correct. The bible fails to mention that God told Adam which day was the first or the seventh.
We actually have the Babylonians to thank for the seven-day week count and the idea that the seventh day is special. But among them, the seventh day was an unlucky day, which is why they didn't do anything important on that day.
>...how do you know which day is the seventh day, when God wants you to rest?
I looked up several references to see why the Adventists keep Sabbath on Saturday. It's confusing. They don't seem to quite precisely answer the question, just stating that God commanded us to observe the seventh day. They don't say why they think Saturday is the seventh day (at least in the web sites I came across). So, by inference, they must think that God started creation on a Sunday/Day 1, which makes Saturday Day 7.
So the interpretation is over whether Sunday is Day 1 or Day 7.
They must have a more specific explanation for their belief - I just didn't come across it on a quick search.
Constantine was a pagan before he decided it was politically expedient to become a Christian. Many pagan holidays and practices were adopted into Christianity. As is explained in the above post that I am responding to, Sunday was an important day of the week to pagans because it was named after the Sun. Constantine was a Sun worshiper before his conversion to Christianity. Worshiping on Sunday was probably one of the pagan practices adopted by Christianity, with the support and influence of Constantine.
The Sabbath was a reminder to Israel of their covenant relationship with God. Basically that if they would keep his law he would be their God and protector. This is implicit in the first word of the command...Remember. It was also a day of spiritual focus to be nearer or sanctified to him. Exodus 31:3.
Christ fulfilled the Law and brought a covenant of grace through the HS. Christians rest in him and worship on Sunday. This is how I understand it.
"Basically that if they would keep his law he would be their God and protector."
Makes me think they accepted that there were so many gods --as anyone looking around a bit in those days would know---and some gods were bullies and so they sought out the one they thought would have their backs and protect them for a price. Sort of like picking which mob boss to give your allegiance (and your money to) to.