1.it is the day on which the religion was perfected and allaah’s favour was completed. 

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-mu’mineen, 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-maa’idah 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 allaah be upon him). it was when he was standing in ‘arafaah on a friday.”

2.it is a day of eid for the people who are in that place.

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.”

3.it is a day by which allaah swore an oath.

the almighty cannot swear by anything except that which is mighty. yawm ‘arafaah is the “witnessed day” mentioned in the aayah (interpretation of the meaning):

“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. 

it is the “odd” [i.e., odd-numbered, witr] by which allaah swore in the aayah (interpretation of the meaning):

“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 dhoo’l-hijjah] and the odd is the day of ‘arafaah [i.e., 9th dhoo’l-hijjah] this is also the view of ‘ikrimah and al-dahhaak.

4.fasting on this day is an expiation for two years.

it was reported from abu qutaadah (may allaah be pleased with him) that the messenger of allaah (peace and blessings of allaah be upon him) was asked about fasting on the day of ‘arafaah. 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 ‘arafaah, because the prophet (peace and blessings of allaah be upon him) did not fast on this day in ‘arafaah. it was narrated that he forbade fasting on the day of ‘arafaah in ‘arafaah.

5.it is the day on which allaah took the covenant from the progeny of adam.

it was reported that ibn ‘abbaas (may allaah be pleased with him) said: the messenger of allaah (peace and blessings of allaah be upon him) said: “allaah took the covenant from the loins of adam in na’maan, 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 allaah, 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 allaah)?’ [al-a’raaf 7:172-173 – interpretation of the meaning].” narrated by ahmad and classed as saheeh by al-albaani. and there is no greater day than this and no greater covenant than this. 

6.it is the day of forgiveness of sins, freedom from the fire and pride in the people who are there:

in saheeh muslim it was narrated from ‘aa’ishah (may allaah be pleased with her) that the prophet (peace and blessings of allaah be upon him) said: “there is no day on which allaah frees more people from the fire than the day of ‘arafaah. he comes close and expresses his pride to the angels, saying, ‘what do these people want?’”

 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.

 and allaah knows best.