9 things you should know about the 9th of Dhul Hijjah:

1. It is called the day of Arafah. (Muslim)
2. Fasting this day expiates the sins of the past and coming year. (Muslim)
3. The Prophet peace be upon him said, “Hajj is Arafah” (Ahmad) i.e. Hajj is invalid if one does not go to Arafah during the prescribed timing.
4. The monumental Prophetic Last Sermon was delivered on this day.
5. Supplication on this day is considered the most virtuous. (Malik; Al-Muwatta)
6. The most meritorious remembrance this day is: {there is no God except Allah.He is One and He has no partner; sovereignty and praise belong to him, and He is powerful over everything}. (Tirmidhi) لا إله إلا الله وحده لا شريك له ، له الملك ، وله الحمد ، وهو على كل شيء قدير
7. With the exception of the battle of Badr, Satan is the most disgraced and humiliated on this day. (Al-Muwatta)
8. There is no other day on which people are emancipated from the Fire more than this day. (Mustadrak)
9. Allah takes an oath of this day in Surah Al-Fajr; Chapter 89