No, this is not Christians observing the Sabbath day. These are Jews observing the Sabbath day in their jewish synagogues and Christians going to Jewish meeting places to teach them about Jesus Christ.
Gentiles knew that the Christians came out to preach to the Jews every Sabbath and wanted to learn the gospel when the Christians came back to preach.
Act 18:4 "And he reasoned in the synagogue every Sabbath, and persuaded both Jews and Greeks."
Act 18:12-13 "When Gallio was proconsul of Achaia, the Jews with one accord rose up against Paul and brought him to the judgment seat, 13 saying, "This [fellow] persuades men to worship God contrary to the law.""
The Context of the scriptures clears up all the questions you posed.
The point you brought up between John 5 and Colossians 2 isn't true to their contexts either. In the context of Colossians 2 it is talking about requirements of the law that we are not under anymore: things regarding eating and drinking, festivals and sabbaths. Chapter 2 isn't about being judged by practicing these things, it's about being judged by NOT practicing these things. The requirements have been wiped away and so has the obligation to observe them. There were people who were still trying to force observations of eating and drinking, and festivals and sabbaths onto new covenant Christians, and Paul was telling them they have no such obligation to observe those things anymore.
Gentiles knew that the Christians came out to preach to the Jews every Sabbath and wanted to learn the gospel when the Christians came back to preach.
Act 18:4 "And he reasoned in the synagogue every Sabbath, and persuaded both Jews and Greeks."
Act 18:12-13 "When Gallio was proconsul of Achaia, the Jews with one accord rose up against Paul and brought him to the judgment seat, 13 saying, "This [fellow] persuades men to worship God contrary to the law.""
The Context of the scriptures clears up all the questions you posed.
The point you brought up between John 5 and Colossians 2 isn't true to their contexts either. In the context of Colossians 2 it is talking about requirements of the law that we are not under anymore: things regarding eating and drinking, festivals and sabbaths. Chapter 2 isn't about being judged by practicing these things, it's about being judged by NOT practicing these things. The requirements have been wiped away and so has the obligation to observe them. There were people who were still trying to force observations of eating and drinking, and festivals and sabbaths onto new covenant Christians, and Paul was telling them they have no such obligation to observe those things anymore.