Given the content of the prophecies of the Hebrew Bible, and the content of the book Revelation, is it possible that the day of the LORD commenced at the writing of the Book of Revelation when John said He was in the Spirit on the day of the Lord? After all, this phrase is used nowhere else in the NT.
It's unlikely that John intended the phrase to refer to the "day of the Lord" as found in the prophets.
While the phrase found in Revelation 1:10 isn't found elsewhere in the New Testament, the phrase "day of the Lord" is found in several places. When the phrase is used elsewhere in the New Testament, the grammar matches that found in the prophets. In 1 Thessalonians 5:2, for instance, the phrase "day of the Lord" is ἡμέρα κυρίου, where κυρίου (Lord) is in the genitive case. The same is true in 2 Peter 3:10. In the LXX, the phrase "day of the Lord" always appears with the genitive case.
In Revelation 1:10, the phrase used is κυριακῇ ἡμέρᾳ, where κυριακῇ is in the dative case and is being used as an adjective. This doesn't rule out the possibility of it referring to the same thing, but it does make it highly unlikely and puts the proof of burden on those who would claim otherwise. Authors tend to retain phraseology when it carries a heavy theological weight.
The context also suggests that John does not intend to refer to the eschatological "day of the Lord" found in the prophets. The phrase in the prophets is accompanied by a dread of expectation and judgement. Yet John's experience, while disturbing, is not shaped after the day of the Lord but after Daniel's experiences with his visions.
Revelation 1:10 says
The Lord's Day is defined as the seventh-day Sabbath in Isaiah 58:13-14
Jesus, the apostles and even Gentile Christians observed the seventh-day Sabbath for a few hundred years after the resurrection (Acts 13:42, Luke 23:55-56, Matthew 28:1, http://www.sabbathtruth.com/ ) This is best proved by Hebrews 4:9 which says
The word rest there is sabbatismos which means a keeping of the Sabbath - http://www.blueletterbible.org/lang/lexicon/lexicon.cfm?Strongs=G4520&t=NKJV
The day of the Lord in the Hebrew Bible sometimes refers to a coming judgment which did not occur on the day that John wrote Revelation...