Why Qiyamat is Called a 'Day'?

There arises a question that when, in the realm of Qiyamat, there will be no day or night, in the absence of the sun and the moon, then what is the meaning of 'The Day of Judgement'? Why is it called a Day? The answer is: We call that part of time a Day in which things become bright because of the light of the sun and thereby human eyes can see those things which are not visible to them during night. Likewise those things, which were hidden from people in the world such as their inner condition, their right or wrong beliefs, their good or bad deeds and their consequences will come to be known to all and their consequences too would become apparent in Qiyamat.

Qiyamat is the Day on which all the secrets and hidden things of man would be unravelled.

And what they never thought of shall become plain to them from Allah. (39:47)

Adapted from the book: "The Hereafter (Ma'aad)" by: "Ayatullah Dastghaib Shiraazi"