The gathering of pilgrims on the 9th day of Dhul Hijjah is known as

Explanation

The Day of Arafat falls on the second day of pilgrimage rituals. At dawn on this day, nearly 2 million Muslim pilgrims will make their way from the town of Mina to a nearby hillside and plain called Mount Arafat and the Plain of Arafat, which is located about 12.5 miles (20 kilometers) from Mecca, the final destination for the pilgrimage.
Every Muslim is expected to make the pilgrimage to once during his or her lifetime. During the afternoon, from about noon until sunset, Muslim pilgrims stand in earnest supplication and devotion, praying for God's abundant forgiveness, and listening to Islamic scholars speak on issues of religious and moral importance. Tears are shed readily as those who gather make repentance and seek God's mercy, recite words of prayer and remembrance and gather together as equals before their Lord. The day closes upon the recitation of the evening prayer of Al-Maghrib.
For many Muslims, the Day of Arafat proves to be the most memorable part of the hajj pilgrimage, and one that stays with them forever.