All praise be to Allaah, may Allaah’s Peace and Blessings be upon His Servant, Prophet Muhammad, the most honorable of all Prophets and Messengers, and upon his family, his Companions, and those who follow them righteously until the Day of Judgment.
It is a fact that the merits of the day of `Arafah (9th of Dul-Hijjah) are great. It is the day when the Muslim Ummah (nation based on one creed) obtained superiority and honor over all Ummahs, as Allaah revealed to His Prophet Muhammad (peace be upon him): This day, I have perfected your religion for you, completed My Favour upon you, and have chosen for you Islaam as your religion. These great words were revealed to Prophet Muhammad while he was in`Arafah on Friday.
Some followers of other religions observed this great merit, for example, the Jews: A Jew said to `Umar (may Allaah be pleased with him): “There is an Aayah (Qur’aanic verse) in your Book; had it been revealed to the Jews, we would have taken the day when it was revealed as a feast.” `Umar said: “Which Aayah?” The Jew said: This day, I have perfected your religion for you Then, `Umar said: “I know the day when it was revealed and the place where it was revealed: It was revealed to the Prophet (peace be upon him) while he was standing on [the mount of] `Arafah, on Friday. Both of them [i.e. the Day of `Arafah and Friday] are Islaamic feasts… read more here.