This day I have perfected

There is day which is most important to the Muslims and this is the Day of Arafat. Why is it so important?
Here are the reasons.

1- It is the day when the deen of Islam was perfected and revelation was completed. Once a Jew came to Umar ibn Al-Khattab and told him: “O Amir al-Mu’meen, there is an ayah in your Book that if it had come to us Jews, we would have taken that day (mentioned in the ayah) as a day of Eid (festival)”. Then Umar asked him “Which ayah?” So the Jew recited it: “Thus day I have perfected your religion for you, completed My favor upon you, and have chosen for you Islam as your religion.” (5:3)

To this Umar replied, “We know on which day and in which place that was revealed to the Prophet (pbuh). It was when standing on Arafah on a Friday.” (Bukhari & Muslim) 

2- It is the “witnessed day”. Allah (swt) says: “By the witnessing day and by the witnessed day” (85:3). The “witnessing” day is Friday and the witnessed day is the day of Arafah according to the sayings of the Prophet (pbuh). Allah (swt) will not swear on anything except that which is highly important and He swore using these two days. 

3- Fasting on this day is expiation of sins for two years.  The Prophet (pbuh) said that it expiates for the sins of the previous year and of the coming year (Muslim). This fasting is recommended for those who are not in pilgrimage. 

4- It is the day when Allah (swt) made an agreement with the descendants of Adam (as).

It was reported that Ibn Abbaas (ra) said: the Prophet (pbuh) said: ‘Allah took the covenant from the …[offspring] of Adam in Na’maan, i.e., Arafah. He brought forth … all his offspring and spread them before Him, and then He addressed them, and said: ‘Am I not your Lord? They said, ‘Yes, we testify,’ in case you should say on the Day of Resurrection: ‘Verily, we have been unaware of this.’ Or in case 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-Baatil (i.e., polytheism and committing crimes and sins, invoking and worshipping others besides Allah)?’ (7:172-173)’ Narrated by Ahmad and classed as saheeh by al-Albaani.
And there is no greater day than this and no greater agreement than this.  

5- The day of Arafah is one of forgiveness and freedom from the hell fire The Prophet (pbuh) had said that there is no day where Allah frees more people from the Fire than on the day of Arafah (Muslim).  On this day Allah is pleased with His servants and asks the angels what the people want from Him, i.e. the people making supplications on Arafah. 

It was reported from Ibn Umar that the Prophet (pbuh) said: ‘Allah expresses His pride to His angels at the time of Isha’ 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 saheeh by al-Albaani.

6- It is the day of Eid for the people who are in that place.
The Prophet pbuh) said: ‘Yawm Arafah (the day of Arafah), 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’.

7- And finally, in the valley of Mount Arafat, the Prophet made his famous Last Sermon.

This was narrated by the authors of al-Sunan.