"Nor will it be permitted for them, so they could make excuses." It may be permitted for them to speak in some situations, and it may not be permitted for them in others; so if it is permitted for them to speak, they will not offer any excuse. Muhammad said: "Yawmu" is recited in the nominative and accusative; whoever puts it in the accusative makes it an adverbial modifier meaning: this threat is for a day, and whoever puts it in the nominative makes this for the day; just as you say, "This is your day."