The day of the sun, or the Lord's day, is not the 7th day sabbath, it was declared by the Catholic Church to supersede the Sabbath and celebrate the resurrection instead.

In saying “the Sabbath . . . has been replaced by Sunday” (CCC 2190), the Church does not dismiss the significance of the Sabbath. The Catechism reminds us, “Sunday is expressly distinguished from the Sabbath which it follows chronologically every week” (CCC 2175).

https://www.catholic.com/magazine/print-edition/the-lords-da...

As for which day is the 7th, you can check with descendants of the good book's authors, still around, and still keeping count. The Jewish faith hasn't lost track.