Adam Clarke on Revelation 1:10

I was in the Spirit - That is, I received the Spirit of prophecy, and was under its influence when the first vision was exhibited.

The Lord's day - The first day of the week, observed as the Christian Sabbath, because on it Jesus Christ rose from the dead; therefore it was called the Lord's day, and has taken place of the Jewish Sabbath throughout the Christian world.

And heard behind me a great voice - This voice came unexpectedly and suddenly. He felt himself under the Divine afflatus; but did not know what scenes were to be represented.

As of a trumpet - This was calculated to call in every wandering thought, to fix his attention, and solemnize his whole frame. Thus God prepared Moses to receive the law. See Exo 19:16, Exo 19:19, etc.