Why do people stand at Arafat on a specific day and not on any other day?

The Day of Standing at Arafat is the ninth of Dhu al-Hijjah. The reason for specifying this day is that Allah's law is divided into two parts: that whose wisdom and benefit are apparent to the intellects, such as the punishments for apostasy, retaliation (qisas), fornication, and alcohol. And that whose wisdom is hidden from the intellects, such as the designation of Ramadan (definition) for fasting, the specification of the five daily prayers, and the designation of Arafat for standing. This is a trial from Allah to ensure submission to Him through obedience, with the certainty that these acts have wisdoms that we have not grasped.