The blowing of the trumpet on the day of qiyaamah.

 The blowing of the trumpet on the day of qiyaamah.

Allah says,

Az-Zumar 39:68

وَنُفِخَ فِي ٱلصُّورِ فَصَعِقَ مَن فِي ٱلسَّمَٰوَٰتِ وَمَن فِي ٱلۡأَرۡضِ إِلَّا مَن شَآءَ ٱللَّهُۖ ثُمَّ نُفِخَ فِيهِ أُخۡرَىٰ فَإِذَا هُمۡ قِيَامࣱ يَنظُرُونَ

And the Horn will be blown, and whoever is in the heavens and whoever is on the earth will fall dead except whom Allāh wills. Then it will be blown again, and at once they will be standing, looking on.

The angel in charge of blowing the horn, Israfeel will blow the first horn and every living thing on earth will die ad the second horn been blown for resurrection

So the horn will be blown twice.

As reported in Hadith,

Abdullah ibn Amr ibn 'As (R.A) said,

The Messenger of Allah (ﷺ) said, "Dajjal (the Antichrist) will appear in my Ummah and he will stay in the world for forty. I do not know whether this will be forty days or forty months or forty years. Allah will then send (Prophet) 'Isa (Jesus), son of Maryam (Mary). 'Isa will pursue him and slaughter him. Then people will survive for seven years (i.e., after the demise of 'Isa) in the state that there will be no rancour between two persons. Then Allah will send a cool breeze from the side of Ash-Sham. None will remain upon the face of the earth having the smallest particle of good or Faith in him but he will die, so much so that even if someone amongst you will enter the innermost part of a mountain, this breeze will reach that place also and will cause him to die. Only the wicked people will survive and they will be as fast as birds (i.e., to commit evil) and as ferocious towards one another as wild beasts. They will never appreciate the good, nor condemn evil. Then Shaitan (Satan) will come to them in the garb of a man and will say: 'Will you not obey me?' They will say: 'What do you order us to do?' He will command them to worship idols. They will have abundance of sustenance and will lead comfortable lives. Then the Trumpet will be blown. Every one hearing it, will turn his neck towards it and will raise it. The first one to hear that Trumpet will be a man who will be busy repairing the basin for hiscamels. He will become unconscious. Allah will send, or will cause to send, rain which will be like dew and there will grow out of it (like wild growth) the bodies of the people. Then the second Trumpet will be blown and they will stand up and begin to look around. Then it will be said: 'O people! Go to your Rubb.' Then there will be a command: 'Make them stand there.' After it they will be called to account. Then it will be said: 'Separate from them the share of the Fire.' It will be asked: 'How much?' It will be said: 'Nine hundred and ninety-nine out of every thousand.' That will be the Day which will make children hoary-headed men because of its terror and that will be the Day when the Shin will be uncovered." [Muslim].