Tafsir Al-Jalalayn: Fussilat, Ayah 12

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. He completed them in the last hour thereof in which He created Adam — which is why He does not say sawā’an ‘evenly’ here as He did earlier; what is said here concords with those verse in which it is stated that the heavens and the earth were created in six days; and in each heaven He revealed its commandment’ that to which He commanded those in it to follow in the way of obedience and worship. And We adorned the lowest heaven with lamps with stars and this was also to guard them hifzan is in the accusative because of its implicit verbal sense in other words ‘We guarded it against the devils lest they try to listen therein to the angels by stealth with meteors’. That is the ordaining of the Mighty in His kingdom the Knower of His creatures.