But you are right....They do claim Sunday is their mark.
from the Convert’s Catechism of Catholic Doctrine by Reverend Peter Giermann.
“Question: Which is the Sabbath day?
Answer: Saturday is the Sabbath day.
Question: Why do we observe Sunday instead of Saturday?
Answer: We observe Sunday instead of Saturday because the Catholic Church in the Council of Laodicea transferred the solemnity from Saturday to Sunday.
Imagine the other things you will find that the Catholic church changed from what God instituted :)