What does Revelation 1:10 mean?

Here, John explains the circumstances surrounding his charge from God: to write to the seven churches. He was somehow "in the Spirit," when he heard a loud voice behind that sounded like a trumpet. This might refer to a state of deep communication with God, or even an overt moment of control by the Holy Spirit.

Some scholars believe that by "Lord's day" John was referring to the tribulation, the seven years of judgments that follow the rapture. It is true that the term "day of the Lord" is used in the Old Testament (Joel 2:1) and 2 Thessalonians 2:2 to refer to the tribulation. However, that does not seem to be what John means in this verse. In 2 Thessalonians 2:2 Paul wrote that the claim that the day of the Lord had already come was false. He declared it was still in the future, and would follow a rebellion—an apostasy—and the appearance of the "man of lawlessness." Most likely, then, John's use of "the Lord's day" implies the first day of the week, when Christians gathered for worship (Acts 20:7).

John writes that he heard a loud voice that sounded like a trumpet. The word for trumpet used here specifically identifies a war trumpet. This would certainly have gotten John's attention. It was definitely a commanding voice that must be heeded.