The day of Arafaah is on 9th dhul hijjah of the Islamic calendar.
It was reported from Ibn Umar that the Prophet (peace and blessings of Allaah be upon him) said: "Allaah expresses His pride to His angels at the time of Ishaa on the Day of Arafaah, about the people of Arafaah. He says, Look at My slaves who have come unkempt and dusty." Narrated by Ahmad and classed as saheeh by al-Albaani.
Another importance of this day is that Allah swore an oath by this day
"By the witnessing day [Friday] and by the witnessed day [the Day of Arafaah]."[al-Burooj 85:3].
It was reported from Abu Hurayrah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said:" The promised day is the Day of Resurrection, the witnessed day is the Day of Arafaah, and the witnessing day is Friday."Narrated by al-Tirmidhi and classed as saheeh by al-Albaani.
Allah says "And by the even and the odd" [al-Fajr 89:3].
Ibn Abbaas said: "The even is the Day of al-Adhaa [i.e., 10th Dhool-Hijjah] and the odd is the Day of Arafaah [i.e., 9th Dhool-Hijjah] This is also the view of Ikrimah and al-Dahhaak.
It was reported that Ibn Abbaas (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 Namaan, i.e., Arafaah. 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, let 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 practised Al-Baatil (i.e., ploytheism and committing crimes and sins, invoking and worshipping others besides Allah)?"[al-Araaf 7:172-173 ]. Narrated by Ahmad and classed as saheeh by al-Albaani.
It Is also the day on which
our Deen was perfected and it is an Eid for those at that place
In Al-Saheehayn it was reported from Umar ibn al-Khattaab (may Allaah be pleased with him) that a Jewish man said to him, "O Ameer al-Mumineen, there is an aayah 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 aayah? "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-Maaidah 5:3 ]. 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 Arafaah on a Friday. "
The Prophet (peace and blessings of Allaah be upon him) said: "Yawm Arafaah (the day of Arafaah), Yawm al-Nahr (the Day of Sacrifice) and Ayyaam al-Tashreeq (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-Khattaab said:
"It i.e., the aayah This day I have perfected was revealed on
a Friday, the Day of Arafaah, both of which praise be to Allaah are Eids for us."
Some people claim that if Arafah falls on Jummah (friday) then that years hajj is equivalent to seventy hajjs. This is false as there is no authentic hadith of the Prophet peace be upon him nor any sayings of the companions stating this.