Q. If the apostles of the New Testament taught that the Gentile believers must observe the Sabbath, why is there no absolute, overt command to do it either from the counsel of Acts 15 or in any of Paul's epistles? Acts 15:20 does not address the Ten Commandments, rather only a very brief comment on some very specific issues with which the Gentiles were struggling. Paul is simply addressing those issues.
By suggesting that he's including the 10 Commandments here would also be leaving out very basic laws such as honoring mothers and fathers and not taking God's name in vain. Most likely, the Gentiles are already were well aware that they must do these things just as they must observe the Sabbath day.
It's interesting, because there was a very real, clear change about circumcision between the old and the new covenant. We hear all about that, but nothing like that with the Sabbath. Through Galatians and Romans and all the writings of Paul, we're constantly being told about circumcision because it was part of the ceremonial laws. So you would think, if just one of the 10 Commandments had been altered or abrogated in any way in the New Testament, that would have been a pinnacle of teaching that they would have tried to be very clear on. But we hear nothing, especially on something as obvious as a day of worship.
For instance, suppose the government should decide we're going to change what side of the street we drive on. If they were not to advertise that thoroughly to the population, can you imagine the traffic problems? Because the right side of the road is an everyday law, just as the Sabbath is an every week law that they lived with. It wasn't an annual feast, and so for the Jews to change it, from the day that went back to Adam, the seventh day, to a new day, to have silence in the New Testament is the loudest argument that it's still intact.