In 1Thessalonians5, the Day of the Lord begins suddenly, like a thief in the night, unexpected, at a time when the world is saying peace and safety.
In 2Thesslaion2:4, the Day of the Lord begins when the son of perdition goes into the temple sits and claims to have achieved God-hood. The transgression of desolation act of Daniel 8:13-14.
So WHAT TRIGGERS THE BEGINNING OF THE DAY OF LORD ?
It will be when the Antichrist, in his stage as the King of Israel, thought-to-be messiah by the Jews, unexpectedly and without warning stops the daily sacrifice of the two lambs, and goes into the temple, to be built following the Gog/Magog event - and sits in the Holy of Holies chamber - reserved for God - and claims himself to have achieved God-hood.
It will end his stage as the King of Israel, thought-to-be messiah by the Jews, as he reveals himself to be the "man of sin" and not the messiah as the Jews will have initially thought.
Don't confuse the transgression of desolation act with the abomination of desolation statue image, which happens later - after the person is killed and comes back to life to become the beast-king of Revelation 13.
The thing to get right first is where does it fit in relation to great tribulation? The following makes it undeniably clear to some of us where it fits in relation to that. But what you brought up per 2 Thessalonians 2:4 though, that is meaning during great tribulation, thus does not involve the day of the Lord, as everything I submit below plainly shows.
Amos 5:18 Woe unto you that desire the day of the LORD! to what end is it for you? the day of the LORD is darkness, and not light.
Amos 5:20 Shall not the day of the LORD be darkness, and not light? even very dark, and no brightness in it?
Now compare with the following.
Matthew 24:29 Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken:
Even though none of these things are meaning in a literal sense, that is beside the point. The point is, what the following is describing---the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven--fit the following to a T---the day of the LORD is darkness, and not light.
Seriously, if the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven--that equals light rather than darkness? Of course not. It equals darkness, the same thing Amos 5:18 and verse 20 indicates the day of the Lord is, that it is darkness not light. And that Matthew 24:29 plainly, thus undeniably, informs us that the day of the Lord is immediately after great tribulation, not prior to it, nor during it instead.
And now that we have that straight, it is then a matter of what initially triggers the day of the Lord after great tribulation?
This for one, meaning the verse below, where it should be plainly obvious to everyone that it is not meaning before or during great trib, thus this passage below agreeing with Matthew 24:29 that the day of the Lord follows great tribulation.
2 Peter 3:10 But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up.
Which then is meaning this in Matthew 24.
Matthew 24:36 But of that day and hour knoweth no man, no, not the angels of heaven, but my Father only.
At this point in Matthew 24, great tribulation is already in the past, since it would be ludicrous to apply this to the beginning of great tribulation rather than the beginning of the day of the Lord.
We also need to factor in the following per 1 Thessalonians 5.
1 Thessalonians 5:2 For yourselves know perfectly that the day of the Lord so cometh as a thief in the night.
3 For when they shall say, Peace and safety; then sudden destruction cometh upon them, as travail upon a woman with child; and they shall not escape.
Here is what I propose.
Revelation 11:9 And they of the people and kindreds and tongues and nations shall see their dead bodies three days and an half, and shall not suffer their dead bodies to be put in graves.
10 And they that dwell upon the earth shall rejoice over them, and make merry, and shall send gifts one to another; because these two prophets tormented them that dwelt on the earth.
This is when they are saying Peace and safety.
But then the following happens soon after.
Revelation 11:11 And after three days and an half the Spirit of life from God entered into them, and they stood upon their feet; and great fear fell upon them which saw them.
12 And they heard a great voice from heaven saying unto them, Come up hither. And they ascended up to heaven in a cloud; and their enemies beheld them.
13 And the same hour was there a great earthquake, and the tenth part of the city fell, and in the earthquake were slain of men seven thousand: and the remnant were affrighted, and gave glory to the God of heaven.
14 The second woe is past; and, behold, the third woe cometh quickly.
15 And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever.
16 And the four and twenty elders, which sat before God on their seats, fell upon their faces, and worshipped God,
17 Saying, We give thee thanks, O Lord God Almighty, which art, and wast, and art to come; because thou hast taken to thee thy great power, and hast reigned.
18 And the nations were angry, and thy wrath is come, and the time of the dead, that they should be judged, and that thou shouldest give reward unto thy servants the prophets, and to the saints, and them that fear thy name, small and great; and shouldest destroy them which destroy the earth.
19 And the temple of God was opened in heaven, and there was seen in his temple the ark of his testament: and there were lightnings, and voices, and thunderings, and an earthquake, and great hail.
And look what verse 18 is involving---and thy wrath is come---exactly what the day of the Lord is involving, His wrath. Then look how this chapter ends--and there were lightnings, and voices, and thunderings, and
an earthquake, and great hail .
Compare that with this in Revelation 16.
Revelation 16:17 And the seventh angel poured out his vial into the air; and there came a great voice out of the temple of heaven, from the throne, saying, It is done.
18 And there were voices, and thunders, and lightnings;
and there was a great earthquake, such as was not since men were upon the earth, so mighty an earthquake, and so great .
19 And the great city was divided into three parts, and the cities of the nations fell: and great Babylon came in remembrance before God, to give unto her the cup of the wine of the fierceness of his wrath.
20 And every island fled away, and the mountains were not found.
21
And there fell upon men a great hail out of heaven , every stone about the weight of a talent: and men blasphemed God because of the plague of the hail; for the plague thereof was exceeding great.