Surah Fussilat 41:12

فَقَضَىٰهُنَّ سَبْعَ سَمَـٰوَاتٍ فِى يَوْمَيْنِ وَأَوْحَىٰ فِى كُلِّ سَمَآءٍ أَمْرَهَا ۚ وَزَيَّنَّا ٱلسَّمَآءَ ٱلدُّنْيَا بِمَصَـٰبِيحَ وَحِفْظًا ۚ ذَٰلِكَ تَقْدِيرُ ٱلْعَزِيزِ ٱلْعَلِيمِ

And He completed them as seven heavens within two days and inspired [i.e., made known] in each heaven its command. And We adorned the nearest heaven with lamps [i.e., stars, for beauty] and as protection. That is the determination of the Exalted in Might, the Knowing.

Translation: Saheeh International

Tafsir

Then He ordained them the suffixed pronoun refers back to al-samā’ ‘the heaven’ because it al-samā’ actually denotes that plural sense to which it will lead in the following clause in other words He made them to be seven heavens in two days — Thursday and Friday. …

Excerpt from Tafsir al-Jalalayn, commentary on 41:12, via Quran.com. The reader has the full commentary and 9 more tafsirs.