first century, by the time Acts was written. Revelation 1:10 references the Lord's Day.
The argument is not that the term does not exist -- the argument is that the NT - first century writers never say it is week-day-1 even though numerous times in the NT text - week-day-1 is mentioned.
But the Bible calls the 7th day the "Holy Day of the LORD" and calls it Sabbath in BOTH the OT AND the NT
here is another Catholic document that mentions the "Lord's Day"
The Catholic Commentary on the Baltimore Catechism post Vatican II - argues the SAME two points.
1965 -- first published 1959
(from "The Faith Explained" page 243
"we know that in the O.T it was the seventh day of the week - the Sabbath day- which was observed as the Lord's day. that was the law as God gave it...'remember to keep holy the Sabbath day.. the early Christian church determined as the Lord's day the first day of the week. That the church had the right to make such a law is evident...
The reason for changing the Lord's day from Saturday to Sunday lies in the fact that to the Christian church the first day of the week had been made double holy...
nothing is said in the bible about the change of the Lord's day from Saturday to Sunday..that is why we find so illogical the attitude of many non-Catholic who say they will believe nothing unless they can find it in the bible and yet will continue to keep Sunday as the Lord's day on the say-so of the Catholic church
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and still another Catholic document
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Convert's Catechism
https://archive.org/stream/convertscatechis00geie/convertscatechis00geie_djvu.txt
3. The Third Commandment.
Q. What is the Third Commandment?
A. The Third Commandment is: Remember
that thou keep holy the Sabbath day.
---------------------------50
Q. Which is the Sabbath day ?
A. Saturday is the Sabbath day.
Q. Why do we observe Sunday instead of Satur-
day ?
A. We observe Sunday instead of Saturday be-
cause the Catholic Church, in the Council of
Laodicea (A.D. 336), transferred the solemnity
from Saturday to Sunday.
Q. Why did the
Catholic Church substitute
Sunday for Saturday?
A.
The Church substituted Sunday for Satur-
day, because Christ rose from the dead on a Sun-
day, and the Holy Ghost descended upon the Apos-
tles on a Sunday.
Q.
By what authority did the Church substitute
Sunday for Saturday?
A. The
Church substituted Sunday for Satur-
day by the plenitude of that divine power which
Jesus Christ bestowed upon her.
Q. What does the Third Commandment com-
mand?
A. The
Third Commandment commands us to
sanctify Sunday as the Lord's Day.