What scripture says that?
The 7th trump of Revelation is the same last trump Paul speaks of because that's when the dead in Christ are raised along with his second coming.
1 Thessalonians 4:16 For
the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and
the dead in Christ shall rise first:
1 Thessalonians 4:17 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
Here we have the second coming associated with a trumpet, and the dead in Christ rising, and the rapture.
1Co 15:51 Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,
1Co 15:52 In a moment, in the twinkling of an eye,
at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.
1Co 15:53 For this corruptible must put on incorruption, and this mortal must put on immortality.
And here we have a trumpet called the last trump, and the dead in Christ being raised. This clearly is at the second coming as the other verse makes clear. The dead in Christ rise and Christ returns. So all we have to do is find out which end times trumpet Christ returns in:
Rev 11:13 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.
Rev 11:14 The second woe is past; and, behold, the third woe cometh quickly.
Rev 11:15
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.
Rev 11:16 And the four and twenty elders, which sat before God on their seats, fell upon their faces, and worshipped God,
Rev 11:17 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.
Rev 11:18 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.
Here we have the end of the Tribulation, and the sounding of the last of the 7 trumps. Though not explicitly mentioned here, I think the language does highly imply Christ's second coming is at this time. The second coming is directly tied to the time for wrath:
Rev 6:15 And the kings of the earth, and the great men, and the rich men, and the chief captains, and the mighty men, and every bondman, and every free man, hid themselves in the dens and in the rocks of the mountains;
Rev 6:16 And said to the mountains and rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and
from the wrath of the Lamb:
Rev 6:17
For the great day of his wrath is come; and who shall be able to stand?
We see this wrath in greater detail here:
Rev 19:11 And I saw heaven opened, and behold a white horse; and he that sat upon him was called Faithful and True, and in righteousness he doth judge and make war.
Rev 19:12 His eyes were as a flame of fire, and on his head were many crowns; and he had a name written, that no man knew, but he himself.
Rev 19:13 And he was clothed with a vesture dipped in blood: and his name is called The Word of God.
Rev 19:14 And the armies which were in heaven followed him upon white horses, clothed in fine linen, white and clean.
Rev 19:15 And out of his mouth goeth a sharp sword, that with it he should smite the nations:
and he shall rule them with a rod of iron: and he treadeth the winepress of the fierceness and wrath of Almighty God.
Rev 19:16 And he hath on his vesture and on his thigh a name written, KING OF KINGS, AND LORD OF LORDS.
Rev 19:17 And I saw an angel standing in the sun; and he cried with a loud voice, saying to all the fowls that fly in the midst of heaven, Come and gather yourselves together unto the supper of the great God;
Rev 19:18 That ye may eat the flesh of kings, and the flesh of captains, and the flesh of mighty men, and the flesh of horses, and of them that sit on them, and the flesh of all men, both free and bond, both small and great.
Rev 19:19 And I saw the beast, and the kings of the earth, and their armies, gathered together to make war against him that sat on the horse, and against his army.
Rev 19:20 And the beast was taken, and with him the false prophet that wrought miracles before him, with which he deceived them that had received the mark of the beast, and them that worshipped his image. These both were cast alive into a lake of fire burning with brimstone.
Rev 19:21 And the remnant were slain with the sword of him that sat upon the horse, which sword proceeded out of his mouth: and all the fowls were filled with their flesh.
So, the "last trump" is the same as the last of the 7 trumpets of Revelation and it is when Christ returns, the dead in Christ rise and the rapture all take place.
Jesus's Second Coming is in Revelation 19. No trumpet in Revelation 19. But it starts of with rejoicing over the demise of Babylon the Great.
Of course there is no trumpet there. It sounds before any Revelation 19 events happen.
It is in the days of the 7th angel sounding that the vials of God's wrath are poured out. During the second half of the Great Tribulation.
The vials are not poured before the 7th trumpet. The verse says the mystery of God is finished in t3h days leading up to the 7th trump, not anything about the vials of wrath.
Also, the Great Tribulation is never said to have a middle, with a first and last part like this. It is 42 months, nothing happens in the middle that is special or written about. It is not known or called a tribulation prior to the 42 months (the tribulation is not 7 years long). God's wrath comes after the Great Tribulation is over, not any moment before that, Revelation 11.
