It is probably meant as the Day of the Lord as mentioned in prophesy. That day of the Lord when the final out pouring of judgments are fulfilled which seems to be what the entire book of Revelation is about.
So what he is saying is "I was translated in spirit into the "day of the Lord" as witnessed by what he recorded.
He was given a panoramic view of the "Lords Day" or Day of the Lord.
I don't believe that the sabbath day was supposed to be changed. It's still Saturday. But I don't believe New Testament believers were told they must observe it as the Jews were used to doing. There are too many texts that say that it is one of the rules that were not to be put on the gentiles. It is clearly stated to be a shadow and the body is Christ. That those who seek to find the rest through the sabbath can't find it, just like those who eat of the brazen altar have no right to eat of the altar that we have.
There is plenty of evidence that they met on the first day of the week but some of those Jewish believers attended synagogue on sabbath as well until they got kicked out for converting Jews to Jesus. Then there is the mention by the elders in Jerusalem that if they wanted to learn about the Jewish laws they could go to the synagogues on the sabbath but they were not going to put that on the gentiles and by making that statement they included the observing of sabbaths.
There is no difference between observing the seventh day sabbath and any other sabbath of Jewish law, they all fall under the same category of a SHADOW.
There is no reference to calling Sunday the "Lord's Day" until over a century later and that seems to be based on a misinterpretation of Rev 1:10 I was in the Spirit on the Lord’s day, and I heard behind me a loud voice like a trumpet
Here is an explanation from Joseph A. Seiss a 19th century Presbyterian Scholar who presents a persuasive case for this to mean the same as the Great Day of the Lord spoken of by the prophets and not Sunday.
He says he “was in Spirit in the Lord’s day,” in which he beheld what he afterwards wrote. What is meant by this Lord’s day? Some answer, Sunday — the first day of the week; but I am not satisfied with this explanation. Sunday belongs indeed to the Lord, but the Scriptures nowhere call it “the Lord’s day.”
None of the Christian writings, for 100 years after Christ, ever call it “the Lord’s day.”
But there is a “Day of the Lord” largely treated of by prophets, apostles, and fathers, the meaning of which is abundantly clear and settled. It is that day in which, Isaiah says, men shall hide in the rocks for fear of the Lord, and for the glory of his majesty;
— the day which Joel describes as the day of destruction from the Almighty, when the Lord shall roar out of Zion, and utter his voice from Jerusalem, and the heavens and the earth shall shake;
— the day to which the closing chapter of Malachi refers as the day that shall burn as an oven, and in which the Sun of Righteousness shall arise with healing in his wings;
— the day which Paul proclaimed from Mars’ Hill as that in which God will judge the world, concerning which he so earnestly exhorted the Thessalonians, and which was not to come until after a great apostasy from the faith, and the ripening of the wicked for destruction;
— the day in the which, Peter says, the heavens shall be changed, the elements melt, the earth burn, and all present orders of things give way to new heavens and a new earth;
— even “the day for which all other days were made.” And in that day I understand John to say, he in some sense was.
In the mysteries of prophetic rapport, which the Scriptures describe as “in Spirit,” and which Paul declared inexplicable, he was caught out of himself, and out of his proper place and time, and stationed amid the stupendous scenes of the great day of God, and made to see the actors in them, and to look upon them transpiring before his eyes, that he might write what he saw, and give it to the Churches.
This is what I understand by his being “in Spirit in the Lord’s day.” I can see no essential difference between hJ Kuriakh hJmera — the Lord’s day, and hJ hJmera Kuriou — the day of the Lord.
They are simply the two forms for signifying the same relations of the same things.
And if John was thus mystically down among the scenes of the last day, and has written only what he says he has written, that is “things that he saw;” it cannot be otherwise but that in dealing with the contents of this book we are dealing with what relates pre-eminently to the great Apocalypse and Epiphany of our Lord, when he cometh to judge the world in righteousness.
And when we come to consider the actual contents of this book, we find them harmonizing exactly with this understanding of its title. It takes as its chief and unmistakable themes what other portions of the Scriptures assign to the great day of the Lord. It is nothing but Apocalypse from beginning to end.
First we have the Apocalypse of Christ in his relation to the earthly Churches, and his judgment of them;
then the Apocalypse of his relation to the glorified Church, and the marshalling of them for his forthcoming to judge the world;
then the Apocalypse of his relation to the scenes of the judgment, as they are manifested on earth under the opening of the seals, the prophesying of the witnesses, and the fall of Babylon;
then the Apocalypse of his actual manifestation to the world in the battle of the great day of God Almighty, the establishment of his kingdom, and the investiture of the saints in their future sovereignties;
and finally the Apocalypse of his relation to the final act of judgment, the destruction of death and the grave, and the introduction of the final estate of a perfected Redemption. What, indeed, is all this, but just what was foretold by all the prophets, by Christ himself, and by all his apostles, as pertaining to THE DAY OF THE LORD? Verily, this book is but the rehearsal, in another and ampler manner, of what all the Scriptures tell us about the last day and the eternal judgment. It is pre-eminently The Apocalypse and Epiphany of Jesus Christ.