Explaining the Day of Decision and what occurs during it

Allah says about the Day of Decision -- and it is the Day of Judgement -- that it is at a fixed time, with a set appointment. Its time cannot be added to or decreased. No one knows its exact time except Allah. This is as Allah says,

وَمَا نُؤَخِّرُهُ إِلاَّ لاًّجَلٍ مَّعْدُودٍ

وَسُيِّرَ‌تِ الْجِبَالُ فَكَانَتْ سَرَ‌ابًا (and the mountains will be set in motion, so they will be as mirage....78:20). The word suyyirat 'set in motion' signifies that mountains, which are taken as an example of firmness and strength, will be shifted from their positions, and will become like little particles of dust flying about in the atmosphere. The word sarab literally means 'to disappear' and it also refers to 'mirage' or an optical illusion, as of a sheet of water, that sometimes appears in a desert, because it seems to be a sheet of water from a distance, but when a person comes near it, it disappears. [ Sihah and Raghib ].