"It has been proved the Sabbath (day?) was still being kept after the cross"? Really? Kept by who? Proven so, where and by who?
Read the book of Acts and Colossians has examples as well.
Acts 13:42
42 So when the Jews went out of the synagogue,[a] the Gentiles begged that these words might be preached to them the next Sabbath.
-The gentiles begged to be preached to the the Sabbath
Acts 13:44
44 On the next Sabbath almost the whole city came together to hear the word of God.
-The whole city came to hear the word on the Sabbath
Acts 15:21
21 For Moses has had throughout many generations those who preach him in every city, being read in the synagogues every Sabbath.
-This says they will learn more each Sabbath as they go along with the 4 basic recommendations that were just given to them
Acts 16:13
13 And on the Sabbath day we went out of the city to the riverside, where prayer was customarily made; and we sat down and spoke to the women who met there.
- Another example of meeting on the Sabbath
Acts 17:2
2 Then Paul, as his custom was, went in to them, and for three Sabbaths reasoned with them from the Scriptures,
- It was Paul's custom and he went to them on multiple Sabbaths
Acts 18:4
4 And he reasoned in the synagogue every Sabbath, and persuaded both Jews and Greeks.
-Both Jew and Gentile meeting on the Sabbath
Colossians 2:16
16 So let no one judge you in food or in drink, or regarding a festival or a new moon or sabbaths,
-This proves it was still being kept because there is no point in telling them not to let anyone judge them for keeping it if it wasnt being kept at all.
Like it or not, the Sabbath was a command of God that they kept before and after the cross. In the 1st century Christianity stated away for the Jews and as time grew on, started substituting things like Saturday with Sunday, Passover with Easter, Christmas and many more RCC traditions. I go to church on Sunday, but im not blind to why I do it anymore, its simply a 1800yr old tradition to meet together and praise God, but not something God command, but he will still honor.