The Roman Emperor Constantine in effect married Christianity with paganism. in an effort to distant himself from the Jewish Sabbath, he introduced his own holy day, in honour of Christ's resurrection from the grave. That day of course, is Sunday.
For a while the church celebrated both Saturday and Sunday as holy days, because Christian Jews also attended the Roman church.
Eventually, Constantine retired from Rome, and left the caring and keeping of the church to the bishop of the church. This individual today, is known as the Roman Catholic Pope.
That is when God's seventh day Sabbath was set aside, and the first day of the week, Sunday, was instituted in its stead. Visit: AdventMessage.com