Tafsir of Al-Infitar 82:17

It means that the matter of the Day of Recompense is such that its true essence cannot be grasped in terms of its terror and severity. However you imagine it, it is beyond that and many times greater. The repetition is for the sake of increasing the sense of dread.

Then, He summarized the state of the people in describing it, saying: {A day when no soul shall possess for another soul anything}, meaning: it will not be able to ward off [harm] from it, nor bring benefit to it in any way, and the command on that day will belong to Allah alone.

• Regarding the grammatical case of Yawm (Day):
  • Whoever reads it in the nominative case (raf‘), it is either in substitution (badal) of "the Day of Recompense," or it is [the predicate of an implied subject], meaning: "It is a day when no soul shall possess..."
  • Whoever reads it in the accusative case (nasb), it is due to an implied verb yudānūn (they are being judged), because "the Recompense" (al-dīn) points to it. Or, it is due to an implied udhkur (remember).
  • It is permissible to read it with a fatha (yawma) because it is annexed to an indeclinable noun, and it remains in the place of the nominative.

"Whoever recites 'Idha al-sama'u infatarat' (When the sky breaks apart), Allah will record for him a good deed for every drop from the sky, and a good deed for every grave."