Fir`awn describes Musa's Proofs as being Magic and Their Agreement to hold a Contest

Allah, the Exalted, informs of what Fir`awn said to Musa when he showed him the great proof. This great sign to Fir`awn was Musa casting down his stick which became a huge snake, and his pulling his hand out from under his arm while it was glowing white without any illness. At this, Fir`awn said, "This is magic that you have brought to us to bewitch us and conquer the people, so that they will follow you. Then you will outnumber us." Fir`awn then said, "Your plan will not work. We have magic just like yours, so do not let yourself be deceived by that what you are doing."

فَاجْعَلْ بَيْنَنَا وَبَيْنَكَ مَوْعِداً

(so appoint a meeting between us and you,) Meaning, `a day that we can come together to present some of our magic to confront yours. It will be at a specified place and time.' With this, Musa said to them,

مَوْعِدُكُمْ يَوْمُ الزِّينَةِ

(Your appointed meeting is the day of festival,) That was the day of their celebration and their New Year's festivity. It was a holiday for them when they took vacation from their work and came together for a large gathering. This day was selected so that all of the people could witness the power of Allah to do whatever He wills. They would see the miracles of the prophets and the futility of magic to contest the supernatural prophetic powers. This is why Musa said,

وَأَن يُحْشَرَ النَّاسُ

(and let the people assemble) meaning all of them.

ضُحًى

(when the sun has risen (forenoon).) meaning in the morning, just before noon. In this way the contest will be most visible, well lit, apparent and obvious in plain view. This is the way of the Prophets. Their work is always clear and apparent. It is never something hidden, or something for sale. This is why he did not say that the meeting should be at night, but rather, it was to be held during the bright part of the day. Ibn `Abbas said, "The day of their festivity was the day of `Ashura'." As-Suddi, Qatadah and Ibn Zayd said, "It was the day of their great celebration." Sa`id bin Jubayr said, "It was the day of their great bazzar." These statements are not contradictory. I say that Allah destroyed Fir`awn and his armies on a day similar to this, just as is confirmed in the Sahih. `Abdur-Rahman bin Zayd bin Aslam said, "It was a flat place where all of the people were on the same level, having an equal view of the event. There was nothing there that would obstruct the view so that some people could see what others did not."