The Reason Why Weekly Sabbath-keeping is Not Mentioned in the NT epistles.
The Weekly Sabbath was a type of Christ. From earliest times, it was always a picture of the "rest" that Christians have in Christ and also a picture of the eternal rest in heaven that is ours as a result of being united to Him. Now that he has come and gone back to heaven we no longer need the type to foreshadow the reality.
The Lord's Day - sunday - the eighth day of the week is not the Christian Sabbath. It is a new day, signifying a New Covanant with new things !
Why honor the old Sabbath while we have the reality - the Lord Jesus Christ ? To honor the old Sabbath is an insult to Him.