(Photo by Mikael Kristenson on Unsplash)
For you know very well that the day of the
Lord will come like a thief in the night.
1 Thessalonians 5:2
The Day of the Lord in 1 Thessalonians 5:1-11 is referring to "an extended period of time, beginning with the tribulation and including the events of the second coming of Christ and millennial kingdom on earth." 
I used to believe the day of the Lord began with the Rapture, moved to the Tribulation, and ended with the second coming of Christ. I understood the Tribulation was a time of judgment and thought the Rapture is what would come like a thief in the night. Now I'm learning that the Rapture is a separate event, and the Day of the Lord is so much more and is what will come like a thief in the night to those who are not ready. The unsaved will be living like the people in the days of Noah were living, thinking nothing will happen even though God had told them through Noah that He was sending a flood to destroy the earth. If they had listened to Noah, they could have escaped God's wrath and got on the boat. But they didn't care. In the end, it will be like that. Before the day of the Lord, people will be completely unaware of what the Bible says because they never cared to listen. They certainly didn't think about God bringing cataclysmic judgment.
BELIEVERS WILL BE GONE
The Thessalonians had already been taught about the day of the Lord. Now they wanted to know how and when the events at the end would take place. Would believers who were still living have to endure the wrath of God? Paul tells them he doesn't really need to write to them about it for it only concerns unbelievers. God didn't appoint them to suffer wrath but to receive salvation through Jesus (1 Thess 5:9). Believers will not be around when the day of the Lord comes because they were previously caught up in the clouds to meet the Lord in the air (1 Thess 4:17). The Day of the Lord is not for the Church. J. Vernon McGee had this to say:
The Lord Jesus does not come to the church like a thief in the night. The church is looking for and waiting for the Lord to come. You don't wait for a thief and look for him and leave a note for him on the back door when you leave your house--"I left the back door open for you, Mr. Thief, and you'll find the silver in the third drawer to the right in the dining room." 
No one does that! The Day of the Lord is for those who aren't expecting Him though, and it will come like a thief in the night. It begins with the Tribulation and the rise of Antichrist. The Church is gone. I suspect people will be given a plausible explanation for the disappearance of so many people, and most will believe it. We know there will be a good number who will become Christians during this time, but the majority will continue in their unbelief. I think of everyone who grew up in the church and heard these truths but didn't accept Christ. Will they be the ones who now believe?
God has chosen not to let anyone know when Jesus will return so that all believers in every age will live with expectancy, knowing it could happen in their lifetime. I believe it can also give us an urgency to share with others the Good News of Jesus Christ!
We are told we are children of the light and the day, not the dark and the night. The night is for sleeping and refers to moral indifference. Night is also when people tend to get drunk and lose self-control. Believers are to stay alert and clearheaded. We are to live protected by our breastplate of faith and love, and wear our helmet, confident in our salvation. Our faith in God protects us inwardly, and our love for those around us protects us outwardly. We stand firm in our salvation, never wavering. We are ready!
Sharing the gospel of Jesus Christ is another way of living ready. Now is the time to share this Good News with others; that Jesus died for our sins and made a way for us to be reconciled to God. This life is the only chance we have to accept what Jesus did for us and be able to escape eternal punishment and receive everlasting life with God in heaven. Jesus is the only One who was perfect as a man. Perfection is what God requires. When Jesus died for us, God accepted His sacrifice as payment for our sins. When we believe that our sin separates us from God, and Jesus is the only way to be saved, God forgives our sins and declares us righteous. He adopts us as His child and guarantees us a place in heaven. He gives us His Spirit to live in us, and help us live the Christian life. We come as we are, for God's salvation is a gift for anyone who believes by faith in what Jesus did. There is no other way!
A NUMBER OF EVENTS
The day of the Lord includes times and seasons. "Times" refers to chronological time and is from the Greek word Chronos. "Seasons" is from the Greek word Kairos and relates to those events that happen at the right or proper time. After the Rapture takes place, a series of events will happen that are all part of the day of the Lord.
the Tribulation begins
the rise of Antichrist
the salvation of the nation Israel
the judgments that come through supernatural means
the return of Jesus Christ
the Battle of Armageddon
the destruction of the nations of the world
the judgment of the sheep and the goats
the establishment of the millennial kingdom
the binding of Satan for the duration of the millennium
the releasing of Satan and his demons for the final rebellion
the Great White Throne Judgment where unbelievers will be judged
the final dispatch of Satan and his angels into the Lake of Fire
the creation of a new heaven and earth
The Day of the Lord comes like a thief in the night for unbelievers. They will be completely taken off guard. Believers will be gone. When we read and study the events that will take place, and possibly in our lifetime, we are more likely to be concerned with others who are not saved. We will want to share the gospel with them and pray for their salvation. We will want to encourage one another and remind each other of why Jesus came.
 Ryrie Study Bible NASB, 1976, note on I Thessalonians 5:2, p. 1809
 J Vernon McGee, Thru the Bible, 1 Corinthians through Revelation, p. 399