Tafsir of Al-Mursalat 77:7

Indeed, that which you are promised—the coming of the Day of Resurrection—is certainly to occur and descend, without doubt. This is the answer to the oath.

Regarding the preceding verses:

• "Extinguished" (ṭumisat): Erased and obliterated. It is said: its light is gone and its essence is destroyed, consistent with the verses "scattered" (intatharat) and "fallen" (inkadarat). It is possible that its light is extinguished, then it is scattered while in a state of being extinguished.
• "Split open" (furrijat): Opened up, becoming gateways. Al-Faraji said: Like the gate of a commander [opening] for them.
• "Blown away" (nusifat): Like grain when it is winnowed with a winnowing fan. Similar to: "And the mountains are ground to dust" (56:5) and "And the mountains become a heap of sand poured out" (73:14). It is also said: snatched quickly from their places, from the expression "I snatched the thing" (intasaftu al-shay’) if you seize it.
• Note: "Extinguished," "split open," and "blown away" have been recited with the shaddah (doubled consonant).

"Appointed" (uqqitat): It has been recited as uqqitat and wuqqitat, with both the shaddah and the light form. The root is the letter waw. The meaning of "appointing the messengers" is the clarification of the time at which they will appear to testify against their nations. "Postponed" (ujmilat) is derived from ajal (term/deadline), just as "appointed" is derived from waqt (time).

"For what Day has it been postponed?" This is an expression of the greatness of that Day and an expression of wonder.

"For the Day of Decision" is an explanation of the "Day of Postponement," which is the Day when judgment is passed between all creatures. The most sound interpretation is that "appointed" (uqqitat) means it has reached its appointed time which it was awaiting: the Day of Resurrection. And "postponed" (ujmilat) means delayed.

If you ask: How can an indefinite noun (nakirah) function as the subject in the phrase: "Woe, that Day, to the deniers"?

I reply: In its origin, it is an accusative verbal noun (maṣdar) acting in place of its verb. However, it was shifted to the nominative case to signify the permanence and continuity of the destruction for the one against whom it is invoked. Similar to: "Peace be upon you" (salāmun ʿalaykum). It is permissible to say "Woe" (waylan) in the accusative, but it has not been recited that way. It is said: "Woe to him, a woe, like a measure."