Paul gives us a reason as to why the Sabbath commandment is not specifically repeated in the New Testament, "For whatever things were written before [in the Old Testament] were written for our learning" (Romans 15:4 Romans 15:4For whatever things were written aforetime were written for our learning, that we through patience and comfort of the scriptures might have hope.
American King James Version×). Thus, the commandment to observe the Sabbath was written for the learning of the New Testament Church, not just for those who lived during the time of the Old Testament.
Paul also wrote this to Timothy, "All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be complete, thoroughly equipped for every good work" (2 Timothy 3:16-17 2 Timothy 3:16-17  All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:
 That the man of God may be perfect, thoroughly furnished to all good works.
American King James Version×). The only scripture they had available at the time Paul wrote this was what we now call The Old Testament. Those Old Testament scriptures clearly commanded the observance of the Sabbath and here Paul upholds that as still being inspired by God and relevant in helping to make us complete.
Also, throughout the book of Acts the Sabbath and Sabbath observances are mentioned and upheld many times (Acts 13:14 Acts 13:14But when they departed from Perga, they came to Antioch in Pisidia, and went into the synagogue on the sabbath day, and sat down.
American King James Version×, Acts 13:42 Acts 13:42And when the Jews were gone out of the synagogue, the Gentiles sought that these words might be preached to them the next sabbath.
American King James Version×, Acts 13:44 Acts 13:44And the next sabbath day came almost the whole city together to hear the word of God.
American King James Version×; Acts 16:13 Acts 16:13And on the sabbath we went out of the city by a river side, where prayer was wont to be made; and we sat down, and spoke to the women which resorted thither.
American King James Version×; Acts 17:2 Acts 17:2And Paul, as his manner was, went in to them, and three sabbath days reasoned with them out of the scriptures,
American King James Version×; Acts 18:4 Acts 18:4And he reasoned in the synagogue every sabbath, and persuaded the Jews and the Greeks.
American King James Version×, Acts 21; Acts 20:6 Acts 20:6And we sailed away from Philippi after the days of unleavened bread, and came to them to Troas in five days; where we stayed seven days.
American King James Version×, Acts 20:16 Acts 20:16For Paul had determined to sail by Ephesus, because he would not spend the time in Asia: for he hurried, if it were possible for him, to be at Jerusalem the day of Pentecost.
American King James Version×; Acts 27:9 Acts 27:9Now when much time was spent, and when sailing was now dangerous, because the fast was now already past, Paul admonished them,
American King James Version×). That first century history from the book of Acts, shows the Sabbath was being observed by the early New Testament Church. Thus, one of the main reasons there was never any need to repeat the command to observe the Sabbath in the New Testament was that in Judea, at the time of Christ, those to whom Christ and the apostles spoke and wrote all kept the Sabbath and realized it was a command of God. Thus, there was never any need to repeat that commandment.
Paul taught Christians "There remains therefore a rest for the people of God" (Hebrews 4:9 Hebrews 4:9There remains therefore a rest to the people of God.
American King James Version×). This word "rest" is translated from the Greek word "sabbatismos" and means "Sabbath rest." The American Standard Version, among several others, translates this correctly.
For much more additional information, you can read online or order your own free printed copy of our study aids "Sunset to Sunset: God's Sabbath Rest" and "The New Covenant: Does It Abolish God's Law?"