The Son of Man is Lord of the Sabbath.
It doesn't matter WHICH day you observe, the Sabbath was made for man, not man for the Sabbath. You are correct about the catholic church and her false authority, but Saturday is not any more correct than Sunday. Two sides of a false argument.
Colossians 2:16, NIV: "Therefore do not let anyone judge you by what you eat or drink, or with regard to a religious festival, a New Moon celebration or a Sabbath day."