"And warn them of the Day of Regret, when the matter has been decided," meaning: when the punishment has been made binding and has fallen upon the people of the Fire.

Yahya: On the authority of a companion of his, from Sufyan, from Salamah ibn Kuhayl, from Abu al-Za'ra', from Abdullah ibn Mas'ud, that he mentioned a hadith concerning the Resurrection. He said: "There is no soul but that it will look at a house in Paradise and a house in the Fire." He said: "It is the Day of Regret, so the people of the Fire will see the house that is in Paradise. He said: Then it will be said to them: 'Had you performed deeds' [you would have attained it]; so regret will seize them. And the people of Paradise will see the house that is in the Fire, and he said: Then it will be said to them: 'Had it not been for Allah's grace upon you' [you would have been there]."