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). 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). 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 13:42, Acts 13:44; Acts 16:13; Acts 17:2; Acts 18:4, Acts 21; Acts 20:6, Acts 20:16; Acts 27:9). 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). 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?"