The Night and Day are Signs of the Great Power of Allah
Allah reminds us of the great signs that He created, including the alternation of the night and day, so that people may rest at night, and go out and earn a living, do their work, and travel during the day, and so that they may know the number of days, weeks, months and years, so they will know the appointed times for paying debts, doing acts of worship, dealing with transactions, paying rents and so on. Allah says:
﴿ لِّتَبۡتَغُواْ فَضۡلاً۬ مِّن رَّبِّكُمۡ ﴾
(that you may seek bounty from your Lord,) meaning, in your living and travels etc.
﴿ وَلِتَعۡلَمُواْ عَدَدَ ٱلسِّنِينَ وَٱلۡحِسَابَۚ ﴾
(and that you may know the number of the years and to count.) If time stood still and never changed, we would not know any of these things, as Allah says:
﴿ قُلۡ أَرَءَيۡتُمۡ إِن جَعَلَ ٱللَّهُ عَلَيۡڪُمُ ٱلَّيۡلَ سَرۡمَدًا إِلَىٰ يَوۡمِ ٱلۡقِيَـٰمَةِ مَنۡ إِلَـٰهٌ غَيۡرُ ٱللَّهِ يَأۡتِيڪُم بِضِيَآءٍۖ أَفَلَا تَسۡمَعُونَ • قُلۡ أَرَءَيۡتُمۡ إِن جَعَلَ ٱللَّهُ عَلَيۡڪُمُ ٱلنَّہَارَ سَرۡمَدًا إِلَىٰ يَوۡمِ ٱلۡقِيَـٰمَةِ مَنۡ إِلَـٰهٌ غَيۡرُ ٱللَّهِ يَأۡتِيڪُم بِلَيۡلٍ۬ تَسۡكُنُونَ فِيهِۖ أَفَلَا تُبۡصِرُونَ • وَمِن رَّحۡمَتِهِۦ جَعَلَ لَكُمُ ٱلَّيۡلَ وَٱلنَّهَارَ لِتَسۡكُنُواْ فِيهِ وَلِتَبۡتَغُواْ مِن فَضۡلِهِۦ وَلَعَلَّكُمۡ تَشۡكُرُونَ ﴾
(Say: "Tell me! If Allah made the night continuous for you till the Day of Resurrection, which god besides Allah could bring you light Will you not then hear'' Say: "Tell me! If Allah made the day continuous for you till the Day of Resurrection, which god besides Allah could bring you night wherein you rest Will you not then see'' It is out of His mercy that He has made for you the night and the day that you may rest therein and that you may seek of His bounty - and in order that you may be grateful.) (28:71-73)
﴿ تَبَارَكَ ٱلَّذِى جَعَلَ فِى ٱلسَّمَآءِ بُرُوجً۬ا وَجَعَلَ فِيہَا سِرَٲجً۬ا وَقَمَرً۬ا مُّنِيرً۬ا • وَهُوَ ٱلَّذِى جَعَلَ ٱلَّيۡلَ وَٱلنَّهَارَ خِلۡفَةً۬ لِّمَنۡ أَرَادَ أَن يَذَّڪَّرَ أَوۡ أَرَادَ شُڪُورً۬ا ﴾
(Blessed be He Who has placed the big stars in the heaven, and has placed therein a great lamp (sun), and a moon giving light. And He it is Who has put the night and the day in succession, for such who desires to remember or desires to show his gratitude.) (25:61-62)
﴿ وَلَهُ ٱخۡتِلَـٰفُ ٱلَّيۡلِ وَٱلنَّهَارِۚ ﴾
(and His is the alternation of night and day.) (23:80)
﴿ يُكَوِّرُ ٱلَّيۡلَ عَلَى ٱلنَّہَارِ وَيُكَوِّرُ ٱلنَّهَارَ عَلَى ٱلَّيۡلِۖ وَسَخَّرَ ٱلشَّمۡسَ وَٱلۡقَمَرَۖ ڪُلٌّ۬ يَجۡرِى لِأَجَلٍ۬ مُّسَمًّىۗ أَلَا هُوَ ٱلۡعَزِيزُ ٱلۡغَفَّـٰرُ ﴾
(He makes the night to go in the day and makes the day to go in the night. And He has subjected the sun and the moon. Each running (on a fixed course) for an appointed term. Verily, He is the All-Mighty, the Oft-Forgiving.) (39:5)
﴿ فَالِقُ ٱلۡإِصۡبَاحِ وَجَعَلَ ٱلَّيۡلَ سَكَنً۬ا وَٱلشَّمۡسَ وَٱلۡقَمَرَ حُسۡبَانً۬اۚ ذَٲلِكَ تَقۡدِيرُ ٱلۡعَزِيزِ ٱلۡعَلِيمِ ﴾
((He is the) Cleaver of the daybreak. He has appointed the night for resting, and the sun and the moon for reckoning. Such is the measuring of the All-Mighty, the All-Knowing.) (6:96),
﴿ وَءَايَةٌ۬ لَّهُمُ ٱلَّيۡلُ نَسۡلَخُ مِنۡهُ ٱلنَّہَارَ فَإِذَا هُم مُّظۡلِمُونَ • وَٱلشَّمۡسُ تَجۡرِى لِمُسۡتَقَرٍّ۬ لَّهَاۚ ذَٲلِكَ تَقۡدِيرُ ٱلۡعَزِيزِ ٱلۡعَلِيمِ ﴾
(And a sign for them is the night. We withdraw therefrom the day, and behold, they are in darkness. And the sun runs on its fixed course for a term (appointed). That is the decree of the All-Mighty, the All-Knowing.) (36:37-38) Allah has made the night a sign having distinguishing features by which it is known. These features include the darkness and the appearance of the moon. The day also has distinguishing features by which it is known; the light and the appearance of the shining sun. He made a distinction between the light of the moon and the light of the sun, so that they may be distinguished from one another, as Allah says:
﴿ هُوَ ٱلَّذِى جَعَلَ ٱلشَّمۡسَ ضِيَآءً۬ وَٱلۡقَمَرَ نُورً۬ا وَقَدَّرَهُ ۥ مَنَازِلَ لِتَعۡلَمُواْ عَدَدَ ٱلسِّنِينَ وَٱلۡحِسَابَۚ مَا خَلَقَ ٱللَّهُ ذَٲلِكَ إِلَّا بِٱلۡحَقِّۚ ﴾
(It is He Who made the sun a shining thing and the moon a light and measured out for it stages that you might know the number of years and to count (periods of time). Allah did not create this but in truth.) (10:5) until,
﴿ لَأَيَـٰتٍ۬ لِّقَوۡمٍ۬ يَتَّقُونَ ﴾
(Ayat for those people who keep their duty to Allah, and fear Him much.) (10:6)
﴿ يَسۡـَٔلُونَكَ عَنِ ٱلۡأَهِلَّةِۖ قُلۡ هِىَ مَوَٲقِيتُ لِلنَّاسِ وَٱلۡحَجِّۗ ﴾
(They ask you about the crescent moons. Say: "These are signs to mark fixed periods of time for mankind and for the pilgrimage.'') (2:189)
﴿ فَمَحَوۡنَآ ءَايَةَ ٱلَّيۡلِ وَجَعَلۡنَآ ءَايَةَ ٱلنَّہَارِ مُبۡصِرَةً۬ ﴾
(Then, We have obliterated the sign of the night (with darkness) while We have made the sign of the day illuminating,) Ibn Jurayj reported that `Abdullah bin Kathir commented on this Ayah: “(It means) the darkness of the night and the twilight of the day.'' Ibn Jurayj reported that Mujahid said: "The sun is the sign of the day and the moon is the sign of the night.
﴿ فَمَحَوۡنَآ ءَايَةَ ٱلَّيۡلِ ﴾
(We have obliterated the sign of the night) this refers to the moon's blackness, which is how Allah has created it.''
﴿ وَجَعَلۡنَا ٱلَّيۡلَ وَٱلنَّہَارَ ءَايَتَيۡنِۖ ﴾
(And We have appointed the night and the day as two Ayat.) Ibn Abi Najih reported that Ibn `Abbas said: "By night and day, this is how Allah created them, may He be glorified.''
﴿ وَكُلَّ إِنسَـٰنٍ أَلۡزَمۡنَـٰهُ طَـٰٓٮِٕرَهُ ۥ فِى عُنُقِهِۦۖ وَنُخۡرِجُ لَهُ ۥ يَوۡمَ ٱلۡقِيَـٰمَةِ ڪِتَـٰبً۬ا يَلۡقَٮٰهُ مَنشُورًا • ٱقۡرَأۡ كِتَـٰبَكَ كَفَىٰ بِنَفۡسِكَ ٱلۡيَوۡمَ عَلَيۡكَ حَسِيبً۬ا ﴾
(13. And We have fastened every man's Ta'irah (deeds) to his neck, and on the Day of Resurrection, We shall bring out for him a Book which he will find wide open. ) (14. (It will be said to him): "Read your Book. You yourself are sufficient as a reckoner against you this Day.'')