Fussilat 41:12

He then formed them into seven heavens in two Days[5] and assigned to each heaven its mandate. And We adorned the lowest heaven with lamps [i.e., stars] which also serve as protection[6]. That is the design of the All-Mighty, the All-Knowing.”

[5] i.e., on Thursday and Friday. So the total period of creating the whole universe was six heavenly Days.
[6] i.e., protection from the devils who try to steal information from the angels in heaven.

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.[1387] That is the determination of the Exalted in Might, the Knowing.

[1387] From the devils who attempt to steal information from the angels.

12. Then He completed and finished from their creation (as) seven heavens in two Days and He made in each heaven its affair. And We adorned the nearest (lowest) heaven with lamps (stars)[1] to be an adornment as well as to guard (from the devils by using them as missiles against the devils). Such is the Decree of Him the All-Mighty, the All-Knower.

[1] (V.41:12) See the footnote of (V.6:97).

Then He ordained them seven heavens in two Days and inspired in each heaven its mandate; and We decked the nether heaven with lamps, and rendered it inviolable. That is the measuring of the Mighty, the Knower.