Surah Ash-Shams 91:4

وَٱلَّيْلِ إِذَا يَغْشَىٰهَا

And [by] the night when it covers [i.e., conceals] it

Translation: Saheeh International

Tafsir

and by the night when it enshrouds her covering her up with its darkness idhā ‘when’ in all three instances is an absolute adverbial operated by the verbal action of the oath.

Excerpt from Tafsir al-Jalalayn, commentary on 91:4, via Quran.com. The reader has the full commentary and 9 more tafsirs.