You are mistaken. The CC believes the Lord's Day was always Sunday, that the Lord's Day refers to the Day Christ rose from the dead. What we did was change the solemnity from the Sabbath to the Lord's Day.
Now the Scriptures alone do not contain all the truths which a Christian is bound to believe, nor do they explicitly enjoin all the duties which he is obliged to practice. Not to mention other examples, is not every Christian obliged to sanctify Sunday and to abstain on that day from unnecessary servile work? Is not the observance of this law among the most prominent of our sacred duties? 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.
—
Faith of Our Fathers, Cardinal Gibbons, p. 72
[36]
Sanctify- set apart as or declare holy; consecrate.
Here is the Cardinal is saying that SCRIPTURE enforces religious observance of Sabbath; A day the Catholic Church NEVER set part or declare as HOLY.
Try as you like it is crystal, unmistakably clear that the RCC does not observe, support or consider Sabbath a day as any different than any other day of the week.
In his opening sentence he declares that the RCC has truth not found in scripture
They should, if the Scripture were their only rule, wash the feet of one another, according to the command of Christ, in the 13th chap. of St. John; —
they should keep, not the Sunday, but the Saturday, according to the commandment, "Remember thou keep holy the SABBATH-day;" for this commandment has not,
in Scripture, been changed or abrogated;... Rev. Stephen Keenan,
A Doctrinal Catechism; New York in 1857, page 101 Imprimatuer
Had she not such power, she could not have done that in which all modern religionists agree with her; —
she could not have substituted the observance of Sunday the first day of the week, for the observance of Saturday the seventh day, a change for which there is no Scriptural authority. Rev. Stephen Keenan,
A Doctrinal Catechism; New York in 1857, page 174
Ask him why he keeps Sunday, and not Saturday, as his day of rest, since he is unwilling either to fast or to abstain. If he reply, that the Scripture orders him to keep the Sunday, but says nothing as to fasting and abstinence, tell him the Scripture speaks of Saturday or the Sabbath, but gives no command anywhere regarding Sunday or the first day of the week. If, then he neglects Saturday as a day of rest and holiness, and substitutes Sunday in its place, and this merely because such was the usage of the ancient Church, should he not, if he wishes to act consistently, observe fasting and abstinence, because the ancient Church so ordained? Rev. Stephen Keenan, A Doctrinal Catechism; New York in 1857, page 181
"The Church, on the other hand, after changing the day of rest from the Jewish Sabbath, or seventh day of the week, to the first,
made the Third Commandment refer to Sunday as the day to be kept holy as the Lord's Day. The Council of Trent (Sess. VI, can. xix) condemns those who deny that the Ten Commandments are binding on Christians." The Catholic Encyclopedia, Commandments of God, Volume IV, © 1908 by Robert Appleton Company, Online Edition © 1999 by Kevin Knight, Nihil Obstat - Remy Lafort, Censor Imprimatur - +John M. Farley, Archbishop of New York, page 153.
''The [Roman Catholic]
Church changed the observance of the Sabbath to Sunday by right of the divine, infallible authority given to her by her founder, Jesus Christ. The Protestant claiming the Bible to be the only guide of faith, has no warrant for observing Sunday. In this matter the Seventh-day Adventist is the only consistent Protestant.'' The Catholic Universe Bulletin, August 14, 1942, p. 4.
The churches own words.... CHANGED the commandment of G-d in their own doing without a shred of scriptural authority to do so.
Some of these statements made by the church as as clear as day. Frankly, Im shocked that you want to try and argue this point. The church is rather openly admitting that Sunday is their own doing and that the Church made this change on their own "authority" that is not found in scripture