The 9th day of Dhul-Hijjah is the day of Arafat. It is this day when the pilgrims gather on the mountain plain of Arafat, praying and supplicating to their Lord.

The day of Arafat holds great importance in Islam since this is the Day when Allah completed his revelation on His Messenger (sallahu alaihe wa-sallam). It is reported in the Saheehayn (i.e. Saheeh al-Bukharee and Saheeh Muslim), from Umar Ibn al-Khattab (radhi allahu anhu) that a Jewish man said to him:

‘O Ameer al-Mumineen (O head of the Muslims)! There is a verse in the Qur’aan, which if was revealed on us, the Jews, we would have taken that day as an Eid (festival).’

Umar (RA) asked:
“Which verse?”

He said:

“This day have perfected your religion for you, completed My favour upon you, and have chosen for you Islam as your religion.”
(Soorah al-Maidah (5): 3)

Umar (radhi allahu anhu) said:

“We know on which day and in which place was this verse revealed to Allah’s Messenger (sallahu alaihe wa-sallam). It was when he was standing in Arafat on a Friday.”

Arafat is the day on which Allah took the covenant from the progeny of Adam (alaihis-salaam), it was reported that Ibn Abbas (radhi allahu anhu) narrated:
“The Messenger of Allah (sallallahu alaihi wa-sallam) related:
“(When Allah created Adam (Alayhis salam) Allah took covenant from him in a place Na’maan on the day of Arafah, then He extracted from him all the descendants who would be born until the end of the world, generation after generation, and He spread them out in front of Him in order to take a covenant from them also. He spoke to them face to face saying:

“Am I not your Lord?”

And they all replied:
‘Yes, we testify to it.”

Allah then explained why He had all of mankind bear witness that He was their Creator and only true God worthy of worship. He said:

“That was in case you (mankind) should say on the day of Resurrection, ‘Surely we were unaware of this. We had no idea that you, Allah, were our Lord. No one told us that we were only supposed to worship you.”…”
[(Saheeh) by Shaikh al-Albanee in Silsilah al-Ahaadeeth as-Saheehah vol: 4, no: 1623]

Indeed, the day of Arafat is a blessed day and there is no other covenant greater than this covenant!

Arafat is a day of Forgiveness from sins, freedom from the Hell-Fire for the people who are present in the plain of Arafat. Aa’ishah (radhi allahu anha) narrated the Messenger of Allah (sallallahu alaihi wa-sallam) saying:

“There is no day on which Allah frees more people from the Fire than the Day of Arafat. He comes close and expresses His pride to the angels saying: “What do these people (the Hajis) want?”
[Saheeh Muslim]

This Haadeeth mentions forgiveness for the pilgrims. In addition to this, fasting on the day of Arafat is a Sunnah and an expiation of sins for the residents.

Hunaydah ibn Khaalid reported from his wife that some of the wives of the Prophet (sallallahu alaihi wa-sallam) said: “The Messenger of Allah (sallallahu alaihi wa-sallam) used to fast on the ninth of Dhul-Hijjah, on the day of Aashoorah, on three days of each month, and on the first two Mondays and Thursdays of each month.” [(Saheeh) by Shaikh al-Albanee in Saheeh Abi Dawood vol: 2, no: 462]

It is reported in Saheeh Muslim that when the Messenger of Allah (sallallahu alaihi wa-sallam) was asked about the fasting on the day of Arafat, he said:

“It Expiates the sins of the previous year and that of the following year.”
[Saheeh Muslim]

This fasting is Mustahabb only for the non-pilgrims and not for the Hajis (the pilgrims) because it was not the practice of Allah’s Messenger (sallallahu alaihi wa-sallam) to fast on the day of Arafat during pilgrimage and in a narration he also forbade doing so. Imaam at-Tirmidhee (rahimahullah) (d. 275H) said: “The People of Knowledge consider it recommended (Mustahabb) to fast on the day of Arafat, except for those at Arafat.”
[Jaami’ut-Tirmidhee (3/377)]