[Pooya/Ali Commentary 83:4] The mighty day is a day of reckoning when the defrauders shall have to answer for every act of theirs which they did to violate the commands of Allah. Those who have been cheated may not know the loss they suffered but the all-knowing Lord knows everything. The record of the wicked is preserved in Sijjin. The root word sijn means "a prison" which suggests that everything done by the wicked is "imprisoned" (recorded) in a register from where nothing can ever escape (be found missing).