However, in favor of the traditional wording is that John uses a different word in the genitive here from what is used in the New Testament for "day of the Lord." In the passage at hand, the Greek reads "te kuriake hemera." In contrast, where the New Testament writers referred to "the day of the Lord," such as in I Thessalonians 5:2, the writers use "he hemera kuriou." "Kuriakos" is used only here and in I Corinthians 11:20 (to refer to "the Lord's Supper"). "Kurios," in contrast, is used 720 times, both for Christ and for higher authorities, in general, which is why it is sometimes translated "master." Thus, the difference is that "kurios" is the more-generic term, while "kuriakos" is used only for Christ, which is why it came into Germanic languages as the word for "church" ("kirk" in Scottish dialect, "kirche" in German), instead of the Greek and Latin "ekklesia" ("eglise" in French, "iglesia" in Spanish, "chiesa" in Italian).
The point is this: the day in Rev. 1:10 is not the Day of the Lord, such as appears not just in I Thessalonians, but also in the Old Testament prophets (e. g., Amos 5:18, Zephaniah 1:14, and Joel 1:15). Rather, it is specifically the day of Christ! That is because the first day was the day of His resurrection (John 20:1ff), and when He revealed Himself glorified to His disciples (John 20:19), and when He sent the Holy Spirit (Acts 2:1 (Pentecost was fifty days after Passover, i. e., seven seventh-days plus one, the first day of the week), and the day that the apostolic church gathered for worship (Acts 20:7 and I Corinthians 16:2). Thus, for John to have been involved in spiritual exercises on that day is only to be expected. To say that he was doing so on "the day of the Lord" makes no sense, at all.
Accordingly, I affirm the statement of the Westminster Confession of Faith (XXI:7): "As it is of the law of nature, that, in general, a due proportion of time be set apart for the worship of God; so, in His Word, by a positive, moral, and perpetual commandment, binding all men in all ages, He hath particularly appointed one day in seven for a Sabbath, to be kept holy unto Him: which, from the beginning of the world to the resurrection of Christ, was the last day of the week; and, from the resurrection of Christ, was changed into the first day of the week, which in Scripture is called the Lord's Day, and is to be continued to the end of the world as the Christian Sabbath." I find the biblical case for the first-day Sabbath, ever since the resurrection of Christ, to succeed over that for a continuing seventh-day Sabbath. I consider the latter to be an effort at judaizing the Christian faith, and therefore to be opposed.