12. And We have appointed the night and the day as two Ayat (signs). Then, We have obliterated the sign of the night (with darkness) while We have made the sign of the day illuminating, that you may seek bounty from your Lord, and that you may know the number of the years and to count (periods of time). And We have explained everything (in detail) with full explanation.

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:

that you may seek bounty from your Lord, meaning, in your living and travels etc.

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:

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)

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. (25:61)

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]

Ayat for those people who keep their duty to Allah, and fear Him much. [10:6]

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]

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.

We have obliterated the sign of the night

this refers to the moon's blackness, which is how Allah has created it."

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."