وَهُوَ الَّذِي جَعَلَ اللَّيْلَ وَالنَّهَارَ خِلْفَةً لِّمَنْ أَرَادَ أَن يَذَّكَّرَ أَوْ أَرَادَ شُكُورًا
«And He is the One who made the night and day to follow each other in succession, for whoever wants to be reminded of Allaah or whoever wants to give thanks for Allaah's favors.»
Shaykh Fawzaan hafizahullaah said:
So the night and the day are a tremendous bounty to be earned for a person who utilizes them in obedience to Allaah, the Mighty and Majestic. The arena for action is the night and the day. You have nothing else besides the night and the day. They are the arena for action, for good, and for pure earning for this world and the Hereafter. So, in the night and in the day, there are lessons and fawaa`id (benefits). Therefore, Allaah swore an oath by al-`Asr.
[Translator's note]:Imaam ibn Jareer at-Tabaree mentions the explanation of this aayah with a chain of narration from ibn `Abbaas (radiyallaahu `anhumaa) and from al-Hasan al-Basree (rahimahullaah) each of them saying the same thing:
And whoever misses out on something during the night, he could reach and catch it during the day, or if he misses out from the day, he could reach and catch it during the night.
[Tranlsated by Abu Talhah rahimahullaah - see lesson 4 of Usool-uth-Thalaathah (audio/text)]