The 9th of Dhu’l Hijjah is known as the Day of Arafah because the pilgrims at Hajj gather at Arafah in worship, which is one of the three central integrals of the Hajj rituals. Its importance is such that the Prophet (Allah bless him and give him peace) said, “Hajj is Arafah.”

The Day of Arafah is important for those not at Hajj as well and it is one of the days when supplications (du`a) are more likely to be accepted

This is based on the Prophet (Allah bless him and give him peace) saying, “The best of supplications are those on the Day of Arafah”