In Revelation 1 we read,
Rev 1:9 I John, who also am your brother, and companion in tribulation, and in the kingdom and patience of Jesus Christ, was in the isle that is called Patmos, for the word of God, and for the testimony of Jesus Christ.
Rev 1:10 I was in the Spirit on the Lord's day, and heard behind me a great voice, as of a trumpet,
Rev 1:11 Saying, I am Alpha and Omega, the first and the last: and, What thou seest, write in a book, and send [it] unto the seven churches which are in Asia; unto Ephesus, and unto Smyrna, and unto Pergamos, and unto Thyatira, and unto Sardis, and unto Philadelphia, and unto Laodicea.
The apostle John says that he was in the Spirit on "the Lord's day". There has been much contention about what "the Lord's day" refers to, in particular, to which day of the week is meant, Saturday or Sunday. What makes answering this question so difficult is that this phrase is used nowhere else in the Bible, so what has occurred is individuals have speculated about what day they think must be in view. In fact, almost without exception, this verse has been interpreted to refer to the day of Christian worship. For this reason, those who worship on Sunday conclude this phrase must refer to Sunday, and of course those who worship on Saturday conclude this phrase must refer to Saturday. But if you read the rationale used for either day, you'll quickly find it is based on a lot of conjecture and very little on actual Bible study, in particular of comparing scripture with scripture. The reality is the Bible does not identify "the Lord's day" to be either Sunday or Saturday (or any other day of the week for that matter), so we must not let our preconceived notions or personal biases cloud any attempt to understand the rightful meaning of the phrase "the Lord's day".
> compare the genitive noun Lord (of the Lord) to the adjective Lord
> compare 1 Cor 11:20, the Lord's supper
> compare "the day of the Lord" to "the Lord's day"
> general context of Rev 1 support