"The Day when secrets are tested," meaning: they shall be examined and laid bare; he implies: the secrets of the hearts.
﴿يَوْم تبلى السرائر﴾ أَيْ: تُخْتَبَرُ وَتُظْهَرُ، يَعْنِي: سَرَائِرَ الْقُلُوب
