وَهُوَ ٱلَّذِى جَعَلَ ٱلَّيْلَ وَٱلنَّهَارَ خِلْفَةً لِّمَنْ أَرَادَ أَن يَذَّكَّرَ أَوْ أَرَادَ شُكُورًا
Wa huwal lazee ja'alal laila wannahaara khilfatal liman araada ai yazzakkara aw araadaa shookooraa
it is He who made the night and day follow each other- so anyone who wishes may be mindful or show gratitude.
And it is he who made the night and the day follow each other, for anyone who wishes to remember or wishes to give thanks.
Khilfah names one thing taking another's place, sharing a stem with khalīfa, so night and day are set up here as mutual replacements — commentators read this to mean that what one of them misses the other stands ready to hold. The offer is left open to anyone who wants it, and the passage that follows fills in the portrait of people who take it up, walking gently and standing through the night.