Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
quran:56:50 –

Are to be gathered together for the appointment of a known Day."  

will be gathered for the tryst, the time, of a known day, that is, the Day of Resurrection.
القرآن:٥٦:٥٠ –

لَمَجْمُوعُونَ إِلَىٰ مِيقَاتِ يَوْمٍ مَعْلُومٍ  

{لمجموعون إلى ميقات} لوقت {يوم معلوم} أي يوم القيامة.

See similar narrations below:

Collected by Qurʾān
quran:26:38 –

So the magicians were assembled for the appointment of a well-known day.  

So the sorcerers were assembled at a fixed time of a known day, which was the morning of the Festival Day.
القرآن:٢٦:٣٨ –

فَجُمِعَ السَّحَرَةُ لِمِيقَاتِ يَوْمٍ مَعْلُومٍ  

{فجمع السحرة لميقات يوم معلوم} وهو وقت الضحى من يوم الزينة.