Fatawalla AAanhum yawma yadAAu addaAAi ila shay-in nukur (surah Al Qamar: 6)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(54:6) So turn away from them, (O Prophet). *5 On the Day when a caller shall call them to a thing most terrible, *6

Fatawalla `anhum; yawma yad`ud daa`i ilaa shai `in nukur

*5) In other words, "Leave them to themselves. When every effort has been made to make them understand the Truth in the most rational ways, when instances have been cited from history to show them the evil results of the denial of the Hereafter, when the dreadful fates suffered by the other nations in consequence of their rejecting the Prophetic messages have been brought to their notice and yet they are disinclined to give up their stubbornness, they should be left alone to gloat over their follies. Now, they would believe only when after death they arise from their graves to see with their own eyes that the Resurrection of which they were being forewarned and exhorted to adopt the truth, had actually Taken place "
*6) Another meaning can be "an unknown thing", a thing which they never could imagine, of which they never had any concept and no idea whatever that it also could occur.