Tafsir of Al-Muzzammil 73:18

السماء منفطر به

Whereon the heaven will be cleft asunder. Al-Hasan and Qatadah both said, "This means, because of it (the Day of Judgement), due to its severity and its horror.'' Then Allah says,

كان وعده مفعوﻻ

His promise is certainly to be accomplished. meaning, the promise of this Day will be fulfilled. This means it will occur and there is no way around it, and it will come to pass and there is no avoiding it