How Many Days Was The Calamity On The Nation Of Aad?
The storm lasted for seven nights and eight days as stated in Quran verse 69:7. In verse 54:19 God is simply highlighting that how unfortunate the day was for the people of Ad when He decided to punish them as a result of their ignoring His warnings.
A related question: Surah Qamar verse 19. اِنَّاۤ اَرۡسَلۡنَا عَلَيۡهِمۡ رِيۡحًا صَرۡصَرًا فِىۡ يَوۡمِ نَحۡسٍ مُّسۡتَمِرٍّۙ‏ ﻿﻿ We sent a tumultuous wind against them on a day of unremitting misfortune. Some very weak traditions also have been cited to support this view, which have further strengthened the idea that this day is ill-omened. For example, there is Ibn Marduyah and Khatib al-Baghdadi’s tradition that the last Wednesday of the month is ill-omened, the ill-omen of which is endless. Ibn Jauzi regards this tradition as fabricated and Ibn Rajab as unauthentic. Hafiz Sakhavi says that all the methods through which this tradition has been reported, are weak. Some narrations in this context are weak, others fabricated. Understood. But what does this word نَحۡسٍ imply? How could a day/time be unfortunate? Considering the wordings of this Hadith especially; Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said, “Allah Almighty said: The son of Adam abuses me. He curses time and I am time, for in my hand are the night and day.” Source: Ṣaḥīḥ al-Bukhārī 4549, Ṣaḥīḥ Muslim 2246