They thought that fortunately there were a lot of skilful...
They thought that fortunately there were a lot of skilful wizards in the vast country of Egypt and if Moses (as) was a wizard, they would gather so many skilful wizards that the secret of Moses (as) would be revealed. The verse says: “(That) they bring to you all skilful sorcerers.” The Arabic word /haširin/ is derived from the word /hašr/ that means to mobilize people for war or suchlike. Thus, now officers had to mobilize wizards at any price to fight Moses (as).
Pharaoh asked people to attend a given place at a particular time, for this gathering and presence would make Moses (as) and Aaron (as) feel alone and might cause the wizards to feel morally boosted. It also would switch on his propaganda machine. Finally, after the proposition of Pharaoh’s companions, some selected officers went to different cities of Egypt and sought everywhere for skilful wizards.
The verse says: “So the sorcerers were got together for the appointment of a day well-known.” In other words, they were already gathered and were made ready for such a day in order to fight Moses (as). As it is understood from the verses of Surah Al-’A‘raf, No.
7, the purpose of the Qur’anic phrase /yaumin ma‘lum/ is one of the feasts of Egyptians which was selected by Moses (as) for fighting, and his aim was to provide people opportunity to attend in that gathering, for he was sure that he would overcome Pharaoh and he wanted to show all people the power of divine signs and weakness of Pharaoh and his companions. He (as) wanted the light of faith to shine in the hearts of more persons.
Then, the next holy verse implies that the people were also invited to attend this struggling, as it declares: “And it was said to the people: ‘Will you assemble?’” This statement shows that Pharaoh’s officers did their jobs very calculatedly and they knew that if they forced people to attend the gathering, they might react negatively, therefore, they declared that if they were willing to attend it, they could do it. Certainly such way of speaking attracted more people to that gathering.