On the Great Day of the Lord’s wrath, God will shield from the firestorm all believers on earth (Isaiah 26:20 Go my people, enter your rooms and shut the doors behind you; hide yourselves for a little while until his wrath has passed by.)

When God's wrath is accomplished, then believers will come out from their hiding places and leap like calves in the ashes. (Malachi 4:2)

Specifically singled out are a select number of extraordinary people…the 144,000.

They have a specific number…144,000, 12,000 from each of all the tribes of Israel. They are not all Jews. They will have the seal of God on their foreheads. It seems that there is a foreshadowing of these 144,000 in the writings of the prophet Ezekiel.

Ezekiel 9:4
and said to him, "Go throughout the city of Jerusalem and put a mark on the foreheads of those who grieve and lament over all the detestable things that are done in it."

In Revelation, the 144,000 are listed as pure and not defiled by women. They will not participate in the activities of the false and apostate church. They will be of a pure heart and mind and that is why they will be selected.

Since the word woman often refers to the church in Biblical analogy, not being defiled by women could mean that these select few will be repulsed by the activities of the false church and absolutely incensed at the abominations that have been committed in God’s name. If it is a mark placed on the forehead it would be conspicuous, and would be something which could at once be recognized designating them as true servants of God. Figuratively speaking, the sealing of the 144,000 could be a protection for them. These exceptional ones will be protected from the wrath of God and the wrath of Satan. Their purpose is to follow Jesus and serve Him personally in the Millennium.

Keep in mind the book of Revelation is not written in chronological order.  Verses 17-20 come first in the timeline and then verses 14 through 16.  There are two separate harvests will be approximately a thousand years apart.

Revelation 14:17-20
 And another angel came out of the temple in Heaven, also having a sharp sickle. And another angel came out from the altar, who had authority over fire. And he spoke with a great cry to him who had the sharp sickle, saying, Thrust in your sharp sickle, and gather the clusters of the vine of the earth, for her grapes are fully ripe. And the angel thrust in his sickle into the earth and gathered the vine of the earth, and cast it into the great winepress of the anger of God. And the winepress was trodden outside the city, and blood came out of the winepress, even to the bridles of the horses, for the space of a thousand, six hundred stadia. 

God the Father will give Jesus the word that the harvest is ripe. The trumpet will sound. Heaven will stand open. Jesus will thunder amid billows of smoke at the head of his vast army of warrior angels as they leave the clouds of heaven. Their numbers will be beyond measure. Their mounts will be white horses, primed and geared for battle. These warrior angels will be dressed in fine linen, white and clean. Jesus as King of Kings and Lord of Lords will be dressed in a robe dipped in blood, with eyes of blazing fire. On His head will be many crowns. He will come with burning anger to execute the wrath of God. (Joel 3:13, Revelation 14:14-16, Matthew 24:14-16, Luke 3:17, Matthew 24:30, Revelation 19:11-15, II Thessalonians 1:5-7, Revelation 2:18, Revelation 19:12, Isaiah 13)

Revelation 14:14-16
I looked, and there before me was a white cloud, and seated on the cloud was one "like a son of man" with a crown of gold on his head and a sharp sickle in his hand. Then another angel came out of the temple and called in a loud voice to him who was sitting on the cloud, "Take your sickle and reap, because the time to reap has come, for the harvest of the earth is ripe." So he who was seated on the cloud swung his sickle over the earth, and the earth was harvested.

This is the end of the Millennium.

For the continuation of this narrative, see the book These Last Days Second Edition.