Allaah (The Most High) said:

وَهُوَ الَّذِي جَعَلَ اللَّيْلَ وَالنَّهَارَ خِلْفَةً لِّمَنْ أَرَادَ أَن يَذَّكَّرَ أَوْ أَرَادَ شُكُورًا

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. [25:62]

The night and the day differ- this one is white and that one is black. So one takes admonition and contemplates on the difference between them and thanks Allaah for them. [Ref 1] Whoever has missed the remembrance of Allaah in the morning, then indeed the evening has arrived so that he can do so. [Ref 2]

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[Ref 1: Source: A paraphrased excerpt from ‘Zaadul Maseer’ by Imaam Ibn Al-Jawzi (rahimahullaah). Publisher: Daar Ibn Hazm 1^st edition 1423AH (Year 2002)]

[Ref 2: Source: A paraphrased excerpt from ‘Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan’ by Imaam Sadi (rahimahullaah). Publisher: Daar ibn Hazm 1^st ed 1424AH (Year 2003)]