It absolutely clear from Scripture that the Mosaic laws were all fulfilled and replaced by Jesus, but many people claim that Sunday is the Christian's Sabbath and that we should not work on that day unless absolutely necessary. Even in the early Church it is clear that idea existed.
Rom 14:5 One person considers one day more sacred than another; another considers every day alike. Each of them should be fully convinced in their own mind.
Rom 14:6 Whoever regards one day as special does so to the Lord. Whoever eats meat does so to the Lord, for they give thanks to God; and whoever abstains does so to the Lord and gives thanks to God.
Sunday is always referred to in the New Testament as "The first day of the week". It is never referred to as "The Lord's Day". That phrase occurs only in
Rev 1:10 On the Lord's Day I was in the Spirit, and I heard behind me a loud voice like a trumpet,
where it specifically says "The Day", i.e. "The Day of the Lord" as described elsewhere in scripture,
Act 2:20 The sun shall be turned into darkness, and the moon into blood, before that great and notable day of the Lord come:
1Co_5:5 To deliver such an one unto Satan for the destruction of the flesh, that the spirit may be saved in the day of the Lord Jesus.
2Co_1:14 As also ye have acknowledged us in part, that we are your rejoicing, even as ye also are ours in the day of the Lord Jesus.
1Th_5:2 For yourselves know perfectly that the day of the Lord so cometh as a thief in the night.
John's use of the term is that he was Spiritually visioning "The Day of the Lord". The use of "The" is often omitted in the Greek (although English translators often add it) so it is used by John to emphasise the vision was for one particular day. There are many Sundays but only one "Day of the Lord". Most certainly the term "The Lord's Day" was never used elsewhere to refer to a Sunday. Of itself, the above reasoning may well not be considered rational, but there is a much stronger basis
Consider
Heb 4:1 Let us therefore fear, lest, a promise being left us of entering into his rest, any of you should seem to come short of it.
Heb 4:2 For unto us was the gospel preached, as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard it.
Heb 4:3 For we which have believed do enter into rest, as he said, As I have sworn in my wrath, if they shall enter into my rest: although the works were finished from the foundation of the world.
Heb 4:4 For he spake in a certain place of the seventh day on this wise, And God did rest the seventh day from all his works.
Heb 4:5 And in this place again, If they shall enter into my rest.
Heb 4:6 Seeing therefore it remaineth that some must enter therein, and they to whom it was first preached entered not in because of unbelief:
Heb 4:7 Again, he limiteth a certain day, saying in David, To day, after so long a time; as it is said, To day if ye will hear his voice, harden not your hearts.
Heb 4:8 For if Jesus (Joshua) had given them rest, then would he not afterward have spoken of another day.
Heb 4:9 There remaineth therefore a rest (NIV and ESV say "Sabbath rest") to the people of God.
Heb 4:10 For he that is entered into his rest, he also hath ceased from his own works, as God did from his.
Heb 4:11 Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief.
This makes it clear that the original Sabbath (when God rested from His works) was a type of the Christian resting from his own works and resting in the finished work of Jesus and we are clearly warned against failing to enter fully into that rest. I have heard some Christians say that the Sabbath Rest of Hebrews refers to our entry into Heaven but that is clearly incorrect because of
Heb 4:11 Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief.
where, addressing born again believers, the writer states it is possible to miss out on the rest. The Rest is not the reliance on the finished work of Jesus for salvation, but for practical daily living out our faith. The "Promised Land" was not a type of Heaven - it had many unbelievers - it was a type of our present life.
But if you do not agree, fine. Make sure your belief is in accordance with
Rom 14:6 Whoever regards one day as special does so to the Lord. Whoever eats meat does so to the Lord, for they give thanks to God; and whoever abstains does so to the Lord and gives thanks to God.