The Lord's Day refers to the end-time events surrounding the return of Jesus Christ, when He will personally and directly intervene in human affairs.
Some people mistakenly assume that, when the apostle John wrote that he "was in the Spirit on the Lord's Day" (Revelation:1:10), he was worshipping on Sunday and experienced the vision on that day. But nowhere does the Bible define "Lord's Day" as the first day of the week. If this were referring to a day of the week, we would have to conclude that John meant the seventh day, since God calls that day His "holy day...the holy day of the LORD" (Isaiah:58:13). Jesus Christ said He was the "Lord of the Sabbath" (Mark:2:28), not some other day of the week (compare Isaiah:58:13).
However, the context of John's vision shows that he wasn't referring to a day of the week at all. Instead, he wrote that the vision transported him forward into that time the Bible elsewhere calls the "day of the LORD," "day of the LORD Jesus Christ" and "day of Christ" (Jeremiah:46:10; Zephaniah:1:14; Acts:2:20; 1 Corinthians:1:8; 5:5; 2 Corinthians:1:14; 1 Thessalonians:5:2; 2 Thessalonians:2:2; 2 Peter:3:10).
These terms are not speaking of a specific 24-hour period. Instead, they refer to the end-time events surrounding the return of Jesus Christ, when He will personally and directly intervene in human affairs. Thus these terms indicate the end of the age of man's rule and the beginning of the age of Jesus Christ. This is the theme of the book of Revelation and "the Lord's Day" John saw in vision.