Fasting and specialty of the day of Arafah (9th day of Dhul Hijjah):

Fasting and specialty of the day of Arafah (9th day of Dhul Hijjah):
The Prophet (sallallahu alayhi wa sallam) said "There is no day that Allah will free people more than the day of Arafah."

There is no day that shaytan is seen more miserable than that day. This is because it is a day that all his prior work goes in vain. People on Arafat are repenting and those not in Hajj are fasting and asking Allah (azza wajal) for forgiveness.

Fasting this day will forgive a year prior to it and and a year after it.

Abu Qatadah reported that the Messenger of Allah (sallallahu alayhi wa sallam) said, "Fasting on the day of 'Arafah is an expiation for two years, the year preceding it and the year following it. Fasting the day of 'Ashurah is an expiation for the year preceding it." [This is related by "the group," except for al-Bukhari and at-Tirmidhi]

Hafsah reported, "There are five things that the Messenger never abandoned: fasting the day of 'Ashurah, fasting the [first] 10 [days of Dhul Hijjah], fasting 3 days of every month and praying two rak'ah before the dawn prayer." [This is related by Ahmad and an-Nasa'i]

For those going to hajj it is better for them not to fast:
Abu Hurairah stated, "The Messenger of Allah forbade fasting on the day of 'Arafah for one who is actually at 'Arafah." [This is related by Ahmad, Abu Dawud, an-Nasa'i, and Ibn Majah]

At-Tirmidhi comments: "The scholars prefer that the day of 'Arafah be fasted unless one is actually at 'Arafah."