Virtues of the Day of Arafah
Question

What are the Virtues of the Day of Arafah?
Praise be to Allah.The virtues of the Day of Arafah are as follows:

It is the day on which the religion was perfected and Allah’s Favour was completed.

In Al-Sahihayn it was reported from ‘Umar ibn al-Khattab (may Allah be pleased with him) that a Jewish man said to him, “O Amir al-Muminin, there is a verse in your Book which you recite; if it had come to us Jews, we would have taken that day as an ‘Eid (festival).” ‘Umar said, “Which verse?” He said: {This day I have perfected your religion for you, completed My favour upon you, and have chosen for you Islam as your religion.} [al-Maidah 5:3 – interpretation of the meaning]. ‘Umar said, “We know on which day and in which place that was revealed to the Prophet (peace and blessings of Allah be upon him). It was when he was standing in ‘Arafah on a Friday.” 

It is a day of ‘Eid for the people who are in that place.

The Prophet (peace and blessings of Allah be upon him) said: “Yawm ‘Arafah (the day of ‘Arafah), Yawm al-Nahr (the Day of Sacrifice ) and Ayyam al-Tashriq (the 3 days following Yawm al-Nahr) are ‘Eid (festival) for us, the people of Islam. These are days of eating and drinking.” This was narrated by the authors of al-Sunan. It was reported that ‘Umar ibn al-Khattab said: “It – i.e., the verse ‘This day I have perfected…’ was revealed on a Friday, the Day of ‘Arafah, both of which – praise be to Allah – are ‘Eids for us.” 

It is a day by which Allah swore an oath.

The Almighty cannot swear by anything except that which is mighty. Yawm ‘Arafah is the “witnessed day” mentioned in the verse (interpretation of the meaning):
“By the witnessing day [Friday] and by the witnessed day [the Day of ‘Arafah].” [al-Buruj 85:3]
Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (peace and blessings of Allah be upon him) said: “The promised day is the Day of Resurrection, the witnessed day is the Day of ‘Arafah, and the witnessing day is Friday.” (Narrated by al-Tirmidhi and classed as sahih by al-Albani) 
It is the “odd” [i.e., odd-numbered, Witr] by which Allah swore in the verse (interpretation of the meaning):
“And by the even and the odd” [al-Fajr 89:3]. Ibn 'Abbas said: “The even is the Day of al-Adha [i.e., 10th Dhu’l-Hijjah] and the odd is the Day of ‘Arafah [i.e., 9th Dhu’l-Hijjah] This is also the view of ‘Ikrimah and al-Dahhak. 

Fasting on this day is an expiation for two years.

Abu Qatadah (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings of Allah be upon him) was asked about fasting on the Day of ‘Arafah . He said, “It expiates for the sins of the previous year and of the coming year.” Narrated by Muslim. 
This (fasting) is mustahabb for those who are not on Hajj. In the case of the one who is on Hajj, it is not Sunnah for him to fast on the Day of ‘Arafah, because the Prophet (peace and blessings of Allah be upon him) did not fast on this day in ‘Arafah. It was narrated that he forbade fasting on the Day of ‘Arafah in ‘Arafah. 

It is the day on which Allah took the covenant from the progeny of Adam.

Ibn 'Abbas (may Allah be pleased with him) said: the Messenger of Allah (peace and blessings of Allah be upon him) said: “Allah took the covenant from the loins of Adam in Na’man, i.e., ‘Arafah. He brought forth from his loins all his offspring and spread them before Him, then He addressed them, and said: Am I not your Lord? They said, ‘Yes, we testify,’ lest you should say on the Day of Resurrection: ‘Verily, we have been unaware of this.’ Or lest you should say: ‘It was only our fathers aforetime who took others as partners in worship along with Allah, and we were (merely their) descendents after them; will You then destroy us because of the deeds of men who practiced Al-Batil (i.e., polytheism and committing crimes and sins, invoking and worshipping others besides Allah)? [al-A’raf 7:172-173 – interpretation of the meaning].” (Narrated by Ahmad and classed as sahih by al-Albani) And there is no greater day than this and no greater covenant than this.  

It is the day of forgiveness of sins, freedom from the Fire and pride in the people who are there:

In Sahih Muslim it was narrated from ‘Aishah (may Allah be pleased with her) that the Prophet (peace and blessings of Allah be upon him) said: “There is no day on which Allah frees more people from the Fire than the Day of ‘Arafah. He comes close and expresses His pride to the angels, saying, ‘What do these people want?’”
Ibn ‘Umar narrated that the Prophet (peace and blessings of Allah be upon him) said: “Allah expresses His pride to His angels at the last part of the day on the Day of ‘Arafah, about the people of ‘Arafah. He says, ‘Look at My slaves who have come unkempt and dusty.’” (Narrated by Ahmad and classed as sahih by al-Albani)
For more about the day of ‘Arafah, please see these answers: 82029 , 32846 , 106587 , 34293 , and 106518
 And Allah knows best.