Q 1: Allah (Glorified and Exalted be He) says in His Noble Book (the meanings of which are translated as):
Allâh it is He Who has created the heavens and the earth, and all that is between them in six Days.
Many Ayahs resembling this one are mentioned in the Qur’an. However, Allah (Blessed and Exalted be He) says in another Ayah (the meanings of which are translated as):
Say (O Muhammad صلى الله عليه وسلم): "Do you verily disbelieve in Him Who created the earth in two Days? And you set up rivals (in worship) with Him? That is the Lord of the ‘Alamîn (mankind, jinn and all that exists).
He placed therein (i.e. the earth) firm mountains from above it, and He blessed it, and measured therein its sustenance (for its dwellers) in four Days equal (i.e. all these four days were equal in the length of time) for all those who ask (about its creation).
Then He rose over (Istawâ) towards the heaven when it was smoke, and said to it and to the earth: "Come both of you willingly or unwillingly." They both said: "We come willingly."
Then He completed and finished from their creation (as) seven heavens in two Days and He made in each heaven its affair. And We adorned the nearest (lowest) heaven with lamps (stars) to be an adornment as well as to guard (from the devils by using them as missiles against the devils). Such is the Decree of Him the All-Mighty, the All-Knower.
What is the relationship between the first and second Ayahs regarding the creation of the Heavens and the Earth in six days and in eight days?
There is no mention of eight days in the above-quoted Ayahs from Surah Fussilat; rather, they are six days only. If you think over this, everything will be clear for you In Sha’a-Allah (if Allah wills). (Part No. 4; Page No. 303) We also recommend you read Tafsir
Ibn Kathir in this regard to clear any ambiguity in your mind about this.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.