1:10 I was in the Spirit on the Lord's day, and I heard behind me a loud voice like the sound of a trumpet,

I was in the Spirit on the Lord's day

John is saying that he was in that special state when a person is entirely engulfed by the Spirit. This Spirit is the same one Jesus said He would send back to Help or Comfort us after He returned to heaven (John 16:7-11). And as Jesus promised, this helper would convict the world concerning sin, righteousness, and judgment. All three are main themes of Revelation.

This "in the Spirit" state also allows movement throughout the spiritual realm. John refers to being transported in this state (Revelation 4:2, Revelation 17:3, and Revelation 21:10). So, too, Paul is transported to the third heaven as he describes in 2 Corinthians 12:1-4.

This "Spirit state" also appears to be a way to receive visions. Paul describes one of his experiences in Acts 22:17-21 where he was given a warning to get out of Jerusalem.

This "Spirit state" is not limited to the New Testament; Ezekiel describes similar transport as it happened to him (Ezekiel 3:12-15 and Ezekiel 37:1-2).

There are several views as to what "the Lord's day" may mean. They are listed here.

1) The first day of the week, that is, Sunday for us.
2) The resurrection day or Easter Sunday.
3) The "day of the Lord" or when the final judgment occurs.

As Paul points out, the day is the Lord's day if you live it for Him (Romans 14:1-8). The important point is that on this day, John was "in the Spirit". And in this state he says,

and I heard behind me a loud voice like the sound of a trumpet,

It is not an actual trumpet, but a voice that sounds like a trumpet. It must grab your attention because John heard this same voice in Revelation 4:1, and yet this same voice takes on the qualities of "many waters" in Revelation 1:15. Note: there are "real" trumpets, seven to be exact, which are used in Revelation to summon a group of judgments upon the earth (Revelation 8:2).