Verse 9 of Surah Fussilat says: "The earth was created in two days" and verse 10 says: "The mountains, blessings and food of the earth were created in four days"; Following these verses, the creation of the heavens is also mentioned in "two days". It may be thought that the sum of these is eight days. But we must note that the two days mentioned in verse 9 are also part of the four days mentioned in verse 10; In this way, in the first two days, the earth was created and in the next two days, other characteristics were added. In fact, verse 9 speaks of the creation of the earth, and verse 10 discusses the total stages of the creation of the earth and its preparation for life.
translated from the Persian website of Ayatullah Naser Makarem Shirazi