The Rapture will occur when…

The Rapture will occur when Jesus Christ returns for his church. This is when all true believers in Christ will be taken from the earth by God into heaven (1 Corinthians 15:51–52; 1 Thessalonians 4:16–17).
The Second Coming will happen when Jesus Christ ​returns to the church to defeat the antichrist, overthrow evil and then establish his thousand-year reign on earth (Revelation 19:11–16).
Comparing the Rapture and the Second Coming of Christ
In the study of Eschatology, these two events are often confused because they are similar. Both happen during end times and both describe a return of Christ. Yet there are important differences to discern. The following is a comparison of the Rapture and the Second Coming of Christ, highlighting the key distinctions noted in Scripture.

1) Meeting in the Air – Versus – Returning with Him
In the Rapture, believers meet the Lord in the air:

For the Lord himself will come down from heaven, with a loud command, with the voice of the archangel and with the trumpet call of God, and the dead in Christ will rise first. After that, we who are still alive and are left will be caught up together with them in the clouds to meet the Lord in the air. And so we will be with the Lord forever. 1 Thessalonians 4:16– 17 (NIV)
In the Second Coming, believers return with the Lord:
The armies of heaven were following him, riding on white horses and dressed in fine linen, white and clean. Revelation 19:14 (NIV)
2) Before Tribulation – Versus – After Tribulation
The Tribulation is a future seven-year period when God will complete his discipline of Israel and final judgment upon the unbelieving citizens of the world. The Rapture will happen before the Tribulation:

For God did not appoint us to suffer wrath but to receive salvation through our Lord Jesus Christ. ​1 Thessalonians 5:9 (NIV)
Since you have kept my command to endure patiently, I will also keep you from the hour of trial that is going to come on the whole world to test the inhabitants of the earth. Revelation 3:10 (NIV)
While the Second Coming will happen at the end of the Tribulation:
And I saw an angel standing in the sun, who cried in a loud voice to all the birds flying in midair, “Come, gather together for the great supper of God, so that you may eat the flesh of kings, generals, and the mighty, of horses and their riders, and the flesh of all people, free and slave,great and small.” Revelation 17-18 (NIV)
3) Deliverance – Versus – Judgment
In the Rapture believers are taken from the earth by God as an act of deliverance:

After that, we who are still alive and are left will be caught up together with them in the clouds to meet the Lord in the air. And so we will be with the Lord forever. 1 Thessalonians 4:13– 17
For God did not appoint us to suffer wrath but to receive salvation through our Lord Jesus Christ. 1 Thessalonians 5:9 (NIV)
In the Second Coming unbelievers are removed from the earth by God as an act of judgment:

Since you have kept my command to endure patiently, I will also keep you from the hour of trial that is going to come on the whole world to test the inhabitants of the earth. Revelation 3:10 (NIV)
But the beast was captured and with it the false prophet who had performed the signs on its behalf. With these signs, he had deluded those who had received the mark of the beast and worshiped its image. The two of them were thrown alive into the fiery lake of burning sulfur. The rest were killed with the sword coming out of the mouth of the rider on the horse, and all the birds gorged themselves on their flesh. Revelation 19:11-21 )NIV)
4) Hidden – Versus – Seen by All
The Rapture, according to Scripture, will be an instantaneous, hidden event:

Listen, I tell you a mystery: We will not all sleep, but we will all be changed— in a flash, in the twinkling of an eye, at the last trumpet. 1 Corinthians 15:50-54
The Second Coming, according to Scripture, will be seen by everyone:

“Look, he is coming with the clouds,” and “every eye will see him, even those who pierced him”; and all peoples on earth “will mourn because of him.” So shall it be! Amen. Revelation 1:7
5) At Any Moment – Versus – Only After Certain Events
The Rapture could happen at any moment:

…While we wait for the blessed hope—the appearing of the glory of our great God and Savior, Jesus Christ, Titus 2:13 (NIV)
For the Lord himself will come down from heaven, with a loud command, with the voice of the archangel and with the trumpet call of God, and the dead in Christ will rise first. 1 Thessalonians 4:14-18 (NIV)
The Second Coming won’t happen until certain events take place:

He will oppose and will exalt himself over everything that is called Godor is worshiped, so that he sets himself up in God’s temple, proclaiming himself to be God. 2 Thessalonians 2:4 (NIV)​
Immediately after the distress of those days, “The sun will be darkened, and the moon will not give its light; the stars will fall from the sky, and the heavenly bodies will be shaken.” Then will appear the sign of the Son of Man in heaven.​ Matthew 24:15-30 (NIV)
Then the kings of the earth, the princes, the generals, the rich, the mighty, and everyone else, both slave and free, hid in caves and among the rocks of the mountains. They called to the mountains and the rocks, “Fall on us and hide us from the face of him who sits on the throne and from the wrath of the Lamb! For the great day of their wrath has come, and who can withstand it?”​ Revelation 6-18 (NIV)