Bismillah.
9th Dhul-Hijjah = Day of `Arafah
10th Dhul-Hijjah = `Ashura
10th Dhul-Hijjah = `Ashura
Narrated Um Al-Fadl (radiallaahu `anhaa):
The people doubted whether the Prophet (sallallaahu `alayhi wasallam) was observing the fast on the Day of 'Arafat, so I sent something for him to drink and he drank it.
It is mustahabb to fast on the ninth and tenth of Dhul-Hijjah as mentioned in a post earlier. But why should we fast on the ninth (Day of `Arafah) if Prophet (sallallaahu `alayhi wasallam) did not fast?
‘Abdullah ibn ‘Abbas (may Allah be pleased with them both) said: ”When the Messenger of Allah (peace and blessings of Allah be upon him) fasted on ‘Ashura’ and commanded the Muslims to fast as well, they said, ‘O Messenger of Allah, it is a day that is venerated by the Jews and Christians.’ The Messenger of Allah (peace and blessings of Allah be upon him) said, ’If I live to see the next year, in sha Allah, we will fast on the ninth day too.’ But it so happened that the Messenger of Allah (peace and blessings of Allah be upon him) passed away before the next year came.” [Reported by Muslim, 1916]
Taken from: http://ilookilisten.wordpress.com/2013/03/17/fasting-on-arafah-day-hadith-no-1420/