"The Sabbath" is Saturday. You may observe "the Lord's Day" on Sunday, but in doing so you are not observing the Sabbath.
It's a New Covenant vs. Old Covenant issue.
The Baltimore Catechism gives:
356. Q. Are the Sabbath day and the Sunday the same?
A. The Sabbath day and the Sunday are not the same. The Sabbath is the seventh day of the week, and is the day which was kept holy in the Old . Law; the Sunday is the first day of the week, and is the day which is kept holy in the New Law.
"Old Law" means the law that God gave to the Jews, the New Law, the law that Our Lord gave to Christians.
A traditionalist Catholic should know the difference between the Sabbath and Sunday.
SD