Surah Ash-Shu'ara 26:38

فَجُمِعَ ٱلسَّحَرَةُ لِمِيقَـٰتِ يَوْمٍ مَّعْلُومٍ

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

Translation: Saheeh International

Tafsir

So the sorcerers were assembled at a fixed time of a known day which was the morning of the Festival Day.

Excerpt from Tafsir al-Jalalayn, commentary on 26:38, via Quran.com. The reader has the full commentary and 9 more tafsirs.