John writes in Revelation 1:10,
I was in the Spirit on the Lord's day, and I heard behind me a loud voice like a trumpet
The phrase "in the Spirit" means here that he was probably in a kind of tantric state, receiving a vision from God. This happened on "the Lord's Day," which is referring to the first day of the week, the day that Christians gathered together for worship. Some argue that the phrase really means "the day of the Lord," which is used throughout Scripture to describe the day of judgment, but the construction in Greek is different here, so "Lord's Day" is a good translation.
The Bible itself doesn't indicate that the first day of the week is called "The Lord's Day," but it is notable that the 2nd-century church continued to meet on the first day of the week, and historical records show that they did call it, "The Lord's Day."
On this Lord's Day, John "heard behind [him] a loud voice like a trumpet." How jarring this must've been. And in verse 11, we read what the voice said:
saying, “Write what you see in a book and send it to the seven churches, to Ephesus and to Smyrna and to Pergamum and to Thyatira and to Sardis and to Philadelphia and to Laodicea.
We'll see in the following verses that this loud voice like a trumpet is that of Christ Himself. And the Savior commands John to write down what He's about to show John in a book and send it to these seven churches.
Let's be reminded at this point that Christ is the head of the church. It is He who rules and reigns over it. It is He who governs it. He has set pastors over churches, but their leadership is delegated. They are not the heads of the church; Christ is. They merely lead as His undershepherds. Remember that your church belongs to Jesus Christ, and it should be serving Him and Him alone. May all that you do together be in service of Him.