[13].Surah Ar-Ra'd [The Thunder]: Ayat 3

Ayat 3. And it is Allah Who spread out the earth, and placed therein firm mountains and rivers and of every kind of fruits He made Zawjain Ithnain (two in pairs - may mean two kinds or it may mean: of two sorts, e.g. black and white, sweet and sour, small and big, etc.) He brings the night as a cover over the day. Verily, in these things, there are Ayat (proofs, evidences, lessons, signs, etc.) for people who reflect.     

Hazrat Abu Hurairah (May Allah be pleased with him) said: The Messenger of Allah  [SAWW](PBUH) took hold of my hand and said, "Allah, the Exalted and Glorious, created the earth on Saturday, the mountains on Sunday, the trees on Monday, the things entailing labour on Tuesday, light on Wednesday, He spread out animals of all kinds on Thursday, and created Adam in the afternoon on Friday, and it was the last hour of Friday between the afternoon and the night.''  [Muslim Book 39, Chapter 02, Hadith # 6707]. 

Lesson : as mentioned above in Surah Ar-Ra'd Ayat 03. "it is Allah Who spread out the earth, and placed therein firm mountains and rivers and of every kind of fruits" What does "Day'' mean, and how long can it be? Its reality only Allah knows. We observe a day of 24 hours. However, the days of the formation of the universe are evidently different from our earthly days. According to the Arabic language, a day may mean just a moment, but also aeons of time. In the Qur'an also, Allah has mentioned that a `day' may be of one thousand or fifty thousand years.