I got this question by e-mail recently:

If Sabbath was not moral law then why does the Roman Catholic Church say it is a mortal sin to miss Sunday [the new Sabbath?] worship?  If we don't have to observe the 7th day then why would we have to observe the first day?  God never ever said that!  I'm so confused.

That's a very good question.
 
The Sabbath is not a moral law in that the timing of it has nothing moral about it - that is purely ceremonial, a remembrance of a certain event.  The moral aspect lies with our duty to God.  It is our duty to worship God, and this includes coming together as a community of believers.  (Obviously not always possible, but the New Testament does call for more than private faith without fellowship.)
 
The New Testament does not bind us to the Sabbath - in fact, Paul tells us that whatever day we keep, we do so for the glory of God.  He also says that the Sabbath is a shadow of what we now have the reality of in Christ.  The day, the timing, is not important to Christians.
 
God did not formally ordain Sunday as a day of worship - that was done by the early Christians, with the Holy Spirit working in them.  Its purpose was to give glory to God by celebrating the resurrection.

Unless there is a good reason not to, refusal to worship with fellow Christians must be seen in a negative light - it is not good for the Christian, and it is not good for the fellowship of the Church.  It is in that sense that not going to church is considered to be a sin.  That is the moral sense that existed when the Sabbath was in force as well, and while the timing on the 7th day is no longer of importance to Christians, the same moral requirement to worship God applies to Christians on Sundays, Easter, Christmas, and any day when the faithful gather together as a group to celebrate some aspect of Christ's life and work.  For those who still keep the 7th day, God will judge their hearts, not their calendar, and so the same moral principle would apply to them regarding the Sabbath.

Michael Scheifler has a “rebuttalâ€? on his website to something I wrote.  He claims that the pope who changed the Sabbath to Sunday was Pope Sylvester I.

In light of the teachings of Ellen White, and in light of history – as taught by real historians – this cannot be seen as more than a failed attempt to make the argument seem viable.  But it is not.

From Ellen White:

"The 1260 years of papal supremacy began with the establishment of the papacy in A. D. 538, and would therefore terminate on 1798." (Great Controversy, p266, 1888 edition.)

"This period, as stated in the preceding chapters, began with the establishment of the papacy, A. D. 538, and terminated in 1798. At that time, when the papacy was abolished and the pope was made captive by the French army, the papal power received its deadly wound, and the prediction was fulfilled, 'He that leadeth into captivity shall go into captivity." (Great Controversy, p439, 1888 edition)

If the papacy was established in 538 AD (EGWs words were changed in later editions to cover up that mistake) then Sylvester could not have been a pope.

Samuele Bacchiocchi writes on the origin of Sunday and Ellen White:

What is problematic is the impression many people get from EGW's statements that the Sabbath was observed "by all Christians . . . in the first centuries" until "the early part of the fourth century [when] the emperor Constantine issued a decree making Sunday a public holiday." (pp. 52-53) ... The earliest documents mentioning Sunday worship go back to Barnabas in 135 and Justin Martyr in 150. Thus, it is evident that Sunday worship was already established by the middle of the second century. This means that to be historically accurate the term "centuries" should be changed to the singular "century." - End Time Issues, #87

More from Ellen White:

"It was on behalf of Sunday that popery first asserted its arrogant claims; and its first resort to the power of the state was to compel the observance of Sunday as 'the Lord's Day.'" (Great Controversy, p447)

"Royal edicts, general councils, and church ordinances sustained by secular power were the steps by which the pagan festival [day of the Sun] attained its position of honor in the Christian world." (Great Controversy, p574)

And Dr Bacchiocchi's rebuttal:

Both statements just cited are inaccurate, because the secular power of the state did not influence or compel Christians to adopt Sunday during the second and third centuries. At that time the Roman emperors were rather hostile toward Christianity. They were more interested to suppress Christianity than to support church leaders in their promotion of Sunday worship. The bishop of Rome could not have resorted to "the power of the state to compel the observance of Sunday as 'the Lord's Day.'" Eventually, beginning with the fourth century, some Roman emperors actively supported the agenda of the church, but this was long after the establishment of Sunday observance. - End Time Issues, #87

Bacchiocchi is presenting a more realistic view of the history of Sunday observance than Ellen White did.  No educated historian takes the claim that the pope change the Sabbath seriously.  The fact is that Sunday was kept by Christians long before Sylvester, long before Constantine.

Ignatius of Antioch, 107 AD: let every friend of Christ keep the Lord's Day as a festival, the resurrection-day, the queen and chief of all the days of the week.
- Epistle to the Magnesians, chp 9. Ante-Nicene Fathers , vol. 1, pg. 62-63.

The Epistle of Barnabas, 70-120 AD: Wherefore we Christians keep the eighth day for joy, on which also Jesus arose from the dead and when he appeared ascended into heaven.
- The Epistle of Barnabas, section 15, 100 AD, Ante-Nicene Fathers , vol. 1, pg. 147

Justin Martyr, 150 AD: But Sunday is the day on which we hold our common assembly, because it is the first day of the week and Jesus our saviour on the same day rose from the dead.
- First apology of Justin, Ch 68

This Pope Sylvester thing is a rather desperate attempt to salvage a claim which should have been abandoned long ago.  All it does today is make people laugh at those who suggest it, and when those who believe it realise they have been duped, they will realise that the whole system is based on such misinformation.  I didn't think that this is what Adventism wants ... but for some, their traditions seem more important than admitting the facts and moving on to a more productive Christianity.