The Day of Arafah is the day on which Muslim Hajj pilgrims gather on Mount Arafah, praying and supplicating to their Lord. Crowds of people stand on Mount Arafah, which is meant to remind those standing of the Day of Resurrection, when crowds of people stand waiting for judgment. Standing as a singular community encourages pilgrims to rid themselves of false pride. [Source : Hajj and Tawheed by Dr. Saleh as-Saleh]
The Day of Arafah is on the ninth day of Dhul-Hijjah, the last month of the Islamic year. Those who are not on the holy pilgrimage (not in Arafah) are highly recommended by Islamic tradition to fast.