وَنُفِخَ فِى ٱلصُّورِ ۚ ذَٰلِكَ يَوْمُ ٱلْوَعِيدِ
The nineteenth verse described the death of one person. The twentieth passes to the raising of all humanity.
This is a choice that recurs in the Qurʾān's account of the hereafter, and its effect is this: the interval between death and resurrection — however long it may be — occupies no room in the telling. For the one who dies, the two are contiguous.
There are two readings as to what the word is — a horn or trumpet and the plural of "form". The first is the reading of the majority and agrees better with the structure of the verb: nufikha fī'ṣ-ṣūr — "it was blown into the trumpet".
And the note set down there holds here too: "I do not speak about the manner of the thing. What the trumpet is, what kind of thing it is, who blows it, is not detailed in the Qurʾān."
In the section on al-Muddaththir a list of the occurrences of this expression in the Qurʾān was given, and Qāf 50/20 was on that list.
What is to be added as belonging to Qāf is that the verb is passive: nufikha — "it was blown". The agent is not stated.
And this stands out when set beside the other verbs of the sūra. Throughout the sūra the verbs are mostly in the first person plural and the agent is explicit: banaynāhā, zayyannāhā, madadnāhā, alqaynā, anbatnā, nazzalnā, aḥyaynā, khalaqnā, naʿlamu, kashafnā.
As was recorded above, the sūra names the day of resurrection in three separate places, and this is the first.
وَعِيد — a threat; treated at the fourteenth verse. And here it is definite: al-waʿīd — "that threat". That is, the one the addressee knows, the one announced to him beforehand.
And the twenty-eighth verse will say this plainly: wa qad qaddamtu ilaykum bi'l-waʿīd — "I sent the threat ahead to you."
The name of the day, that is, comes not from what will happen on that day but from the word spoken before it.
I give this as my own reading: the day is named not by a surprise but by the keeping of a word that was given.