Not for salvation since the first gentiles received the holy spirit without it.
Christians are not the same as Jews. I think this is at the root of your problems with the Lord’s Day.
Sabbatarians tend to identify with what were called Judaizers. Christianity split from Judaism, and it was not a friendly parting, St. Paul was right in the middle of it.
Paul was certainly a follower of his Lord and savior Jesus Christ. The position you're espousing would mean that he was a liar when he spoke multiple times of how he followed Christ, tradition and scripture.
There was certainly a parting of traditional Christianity away from Judaism, but there was also a parting from the clear and direct teachings of the Lord Jesus Christ and his disciples. This parting was due to anti-Jewish sentiment in the Roman empire due to a series of Jewish revolts against Rome. These revolts and the resultant anti-semitism caused some who called themselves Christians to STOP observing biblical practices for fear of being persecuted.
But..but.. Christ practiced circumcision! Where did he change that?
Surely Paul didn’t teach against Christ’s teaching.
Your interpretation must be wrong.
:)