There is no New Testament directive anywhere to keep a sabbath day. After the cross we read nothing of Jesus EVER keeping the sabbath.
Paul only preached in synagogues on the sabbath because that is where the Jews, whom he sought to reach, were in one place together at a time to effectively preach to them all, and Paul had access to the synagogues at that time.
They only observed sabbath when they were with other Jews.
After
Acts 18 nothing is said about sabbath day.
Every day belongs to the Lord. In fact sabbath helps the error that we serve God one day a week and then do what we want the rest of the week. lol
There is no sabbath day now. It was a law thing.