If the Sabbath is Saturday and it's known that the Roman Catholic Church changed the day week to worship from Saturday to Sunday, why then would Protestants continue to worship on Sunday? I understand that Protestantism is a large group - I believe all of the sub-denominations do this for the same reason. If there are different reasons per denomination, I'll either change the question or make a question for each Protestant Denomination.
Cardinal James Gibbons, The Faith of Our Fathers (Ayers Publishing, 1978)
But you may read the Bible from Genesis to Revelation, and you will not find a single line authorizing the sanctification of Sunday. The Scriptures enforce the religious observance of Saturday, a day which we never sanctify.
John A. O'Brien, The Faith of Millions: the Credentials of the Catholic Religion Revised Edition (Our Sunday Visitor Publishing, 1974)
But since Saturday, not Sunday, is specified in the Bible, isn't it curious that non-Catholics, who claim to take their religion directly from the Bible and not from the Church, observe Sunday instead of Saturday? Yes, of course, it is inconsistent; but this change was made about fifteen centuries before Protestantism was born, and by that time the custom was universally observed. They have continued the custom even though it rests upon the authority of the Catholic Church and not upon and explicit text in the Bible. That observance remains as a reminder of the Mother Church from which the non-Catholic sects broke away—like a boy running away from home but still carrying in his pocket a picture of his mother or a lock of her hair.