Virtues of the Day of Arafah:
⁃ 2 years worth of (minor) sins are expiated
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 and he said: He was asked about fasting on the day of 'Arafa (9th of Dhu'I-Hijja), whereupon he said: It expiates the sins of the preceding year and the coming year. [Sahih Muslim 1162b]
⁃ Day of forgiveness from sins and freedom from the fire.
A’isha (May Allah be pleased with her) reported: The Messenger of Allah, peace and blessings be upon him, said, “There is no day upon which Allah frees more of His servants from the Hellfire than the day of Arafat. He draws near and then he boasts of them to the angels, saying: What do these servants want?” [Sahih Muslim 1348].
As for you staying till the evening in Arafah, then Allah descends to the sky of the Dunya and He boasts about you to the Angels, and says: ‘My slaves have come to Me, looking rough, from every deep valley hoping for My mercy, so if your sins were equivalent to the amount of sand or the drops of rain or like the foam on the sea I will forgive them. So go forth My slaves! Having forgiveness and for what or who you have interceded for.” [Tabrani, Al-Albani graded it Hasan, Taken from ‘Saheeh al-Targheeb wa Tarheeb’. Volume 2,Page 9-10, hadeeth no. 1112].
Abdullah ibn Umar reported: The Prophet, peace and blessings be upon him, said, “Verily, Allah Almighty boasts to His angels of the pilgrims on the afternoon of the day of Arafat, saying: Look at My servants, coming to Me disheveled and dusty.” [Musnad Ahmad 7089, classed as Sahih by Al-Albani]
⁃ Best Dua is to say it on this day
‘Amr ibn Shu’ayb reported: The Prophet, peace and blessings be upon him, said, “The best supplication is that which is made on the day of Arafat. The best of it is what was said by myself and the prophets before me: There is no God but Allah alone, without any partners, unto Him belong the dominion and all praise and He has power over all things.” [Tirmidhi 3585, graded as Hasan by Al-Albani]
The arabic for the dua:
لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُو عَلَى كُلِّ شَيْءٍ قَدِيرٌ
Transliteration: La ilaha illallahu wahdahu la sharika lahu lahul mulku wa lahul hamdu wa huwa ala kulli shay in qadeer
Talhah ibn ‘Ubayd ibn Kurayz narrated in a mursal report: “The best of du’a is du’a on the day of ‘Arafah.” [Narrated by Malik in al-Muwatta’ (500); classed as hasan by al-Albani in Sahih al-Jami’ (1102)]
⁃ Allah perfected the religion on this day.
Imam Ahmad recorded that Tariq bin Shihab said, "A Jewish man said to `Umar bin Al-Khattab, `O Leader of the Believers! There is a verse in your Book, which is read by all of you (Muslims), and had it been revealed to us, we would have taken that day (on which it was revealed) as a day of celebration.' `Umar bin Al-Khattab asked, `Which is that verse' The Jew replied (This day, I have perfected your religion for you, completed My favor upon you...) `Umar replied, `By Allah! I know when and where this verse was revealed to Allah's Messenger . It was the evening on the Day of `Arafah on a Friday."' Al-Bukhari recorded this Hadith through Al-Hasan bin As-Sabbah from Ja`far bin `Awn. Muslim, At-Tirmidhi and An-Nasa'i also recorded this Hadith.