Many Pretribulationalists have wrongly assumed that the six seals are God’s wrath and thus part of the Day of the Lord. This is simply assumed and necessary for the Pretrib system because they cannot allow any prophesied events to unfold before the rapture because that would undermine their Holy Grail: Imminency.
The seals are not God’s wrath but precursors to the Day of the Lord. The following is not meant to be an exhaustive explanation of this supposition, but an outline of points.
First, the Cosmic Disturbances of the sixth seal announce God’s wrath upon the ungodly, “The sun will be turned to darkness and the moon to blood before the coming of the great and dreadful day of the LORD.” (Joel 2:31)
Second, both the responses from the martyrs in the fifth seal and the ungodly in the sixth seal reveal that God’s wrath is soon to come–they have not viewed it already in the past. The fifth seal martyrs are asking when God will pour out his wrath; and the ungodly in the sixth seal are running to the caves and crying out about the impending wrath.
Third, the fact that there are two groups of people, one group being sealed and the other delivered, just before the seventh seal suggests strongly that they are being sealed and delivered from something looming that will come upon the whole world.
Fourth, the nature of the events in the first four seals are “natural” (but intense) catastrophes (wars, famines, etc.) carried out by “horsemen.” This is in contrast to when the seventh seal (supernatural contents) is opened up and the unmistakable wrath of God is mediated directly by angels via the trumpet and bowl judgments against the ungodly. (By the way, a fourth of mankind is not killed in the fourth seal; there is only, “power over a fourth of the earth.”)
Fifth, the first five seals in Revelation chapter six parallel Jesus’ teaching about this being the “beginning of birth pangs” in Matthew 24:5-8.
Sixth, obviously the fifth seal is not God’s wrath because it specifically speaks of the martyrdom of believers. And since believers are promised protection from the Day of the Lord’s wrath, to argue that the fifth seal is God’s wrath is contradictory.
Seventh, when the seventh seal is opened, immediately it says that there was silence in heaven for about a half an hour. The most plausible explanation for this silence is for all the members of the heavenly court to observe the grave and profound significance of the event that is to follow: The Eschatological Day of the Lord’s wrath. Immediately after this silence it says,

“Then the angel took the censer, filled it with fire from the altar, and hurled it on the earth; and there came peals of thunder, rumblings, flashes of lightning and an earthquake.”

This unprecedented dreadful opening act of God’s wrath is then followed by how the Old Testament prophets characterized the Day of the Lord: fire and destruction. And this is realized in the trumpet and bowl judgments.