Tafsir of Qaf 50:20

(And the Trumpet shall be blown): That is, the time of the Resurrection. The demonstrative pronoun ("that") is an indication of the blowing understood from the verb "blown," and the phrasing implies an omitted genitive addition (mudaf); meaning: the time of that blowing is (the Day of the Threat).

(That is, the day the threat is fulfilled): The threat that took place in this world, or the day the threat occurs, on the basis that it is an expression for the promised punishment. It has been permitted that the demonstrative refers to the time understood from "blown," for just as a verb indicates an action, it also indicates time; according to this, there is no need to assume anything [as an omitted word]. However, it has been objected that an indication referring to the time of an action is something without parallel. Furthermore, the specific mention of the "threat" is for the sake of intimidation, assuming the address is to man in an absolute sense, even though it is also the Day of the Promise for him.