مَوْعِدُكُمْ يَوْمُ الزِّينَةِ وَأَن يُحْشَرَ النَّاسُ ضُحًى (Your appointment is the festival day and that the people are assembled at forenoon - 20:59). يَوْمُ الزِّينَةِ has been explained differently by different authorities. Some say it was a special festival when the Egyptians, attired in beautiful dresses, gathered outside the towns, while others say that it was a Saturday (' یوم السَّبت), and according to some others it was the tenth day of Muharram (عاشُورا).
The great advantage in fixing for the contest
It was a wise move on the part of Sayyidna Musa (علیہ السلام) to fix festival day as the day of contest when all Egyptians, high and low, were expected to assemble at an appointed place. There was also a point in fixing early forenoon as the time for the contest because this is the time when people, having finished their daily chores, are free to engage in other things. Also daylight and visibility are at their best in the early forenoon and people, while dispersing after witnessing a momentous event, spread the news far and wide. Thus when, with Allah's help, Sayyidna Musa (علیہ السلام) inflicted a crushing defeat on the Egyptian magicians, the story became known the same day to people living in far flung places.
Magic - its truth, forms and the rules governing it
For a detailed discussion on this subject reference may be made to the story of Harut and Marut in Surah Al-Baqarah at page 265 to 278 of Ma'ariful Qur'an vol. I.