فَلَ أُقْسِمُ بِرَبِّ الْمَشَارِقِ وَالْمَغَارِبِ
But no!
I swear by the Lord of the easts and the wests.
meaning, the One Who created the heavens and the earth and made the east and the west. He is the One Who subjected the stars causing them to appear in the eastern parts of the sky and vanish in the western parts of it.
The point of this statement is that the matter is not as the disbelievers claim;that there is no final return, no reckoning, no resurrection and no gathering. Rather all of this will occur and come to pass. There is no way of avoiding it. This is the reason that Allah has stated a negation at the beginning of this oath. This shows that He is swearing by a denial of their claim. This is a refutation of their false claim of rejecting the Day of Judgement. They already witnessed the greatness of Allah's power in what is more convincing than the Day of Judgement, that is the creation of the heavens and the earth, and the subjection of the creatures in them, the animals, the inanimate objects and the other types of creatures that exist.
This is why Allah says,
لَخَلْقُ السَّمَـوَتِ وَالاٌّرْضِ أَكْـبَرُ مِنْ خَلْقِ النَّاسِ وَلَـكِنَّ أَكْـثَرَ النَّاسِ لَا يَعْلَمُونَ
The creation of the heavens and the earth is indeed greater than the creation of mankind; yet, most of mankind know not. (40;57)
Allah also says,
أَوَلَمْ يَرَوْاْ أَنَّ اللَّهَ الَّذِى خَلَقَ السَّمَـوَتِ وَالاٌّرْضِ وَلَمْ يَعْىَ بِخَلْقِهِنَّ بِقَادِرٍ عَلَى أَن يُحْىِ الْمَوْتَى بَلَى إِنَّهُ عَلَى كُلِّ شَىْءٍ قَدِيرٌ
Do they not see that Allah, Who created the heavens and the earth, and was not wearied by their creation, is able to give life to the dead?
Yes, He surely is Able to do all things. (46;33)
Allah says in another Ayah,
أَوَلَـيْسَ الَذِى خَلَقَ السَّمَـوتِ وَالاٌّرْضَ بِقَـدِرٍ عَلَى أَن يَخْلُقَ مِثْلَهُم بَلَى وَهُوَ الْخَلَّـقُ الْعَلِيمُ
إِنَّمَأ أَمْرُهُ إِذَا أَرَادَ شَيْياً أَن يَقُولَ لَهُ كُن فَيَكُونُ
Is not He Who created the heavens and the earth, able to create the like of them?
Yes, indeed! He is the All-Knowing Supreme Creator.
Verily, His command, when He intends a thing, is only that He says to it, "Be!"-- and it is! (36;81,82)
So here He says,
فَلَ أُقْسِمُ بِرَبِّ الْمَشَارِقِ وَالْمَغَارِبِ إِنَّا لَقَادِرُونَ