Riyad as-Salihin (The Meadows of the Righteous)
by Imam Nawawi
227. Chapter: On the excellence of fasting the Day of ‘Arafa, ‘Ashura’ and the ninth of Muharram
1250. Abu Qatada said “The Messenger of Allah, may Allah bless him and grant him peace, was asked about fasting the Day of ‘Arafat. He said, ‘It expiates the past year and coming year.'” [Muslim]
1251. Ibn ‘Abbas reported that the Messenger of Allah, may Allah bless him and grant him peace, fasted the Day of ‘Ashura’ and ordered it to be fasted. [Agreed upon]
1252. Abu Qatada reported that the Messenger of Allah, may Allah bless him and grant him peace, was asked about fasting the Day of ‘Ashura’ and said, “It expiates the past year.” [Muslim]
1253. Ibn ‘Abbas reported that the Messenger of Allah, may Allah bless him and grant him peace, said, “If I remain alive until next year, I will fast the ninth.” [Muslim]
and from Wikipedia:
Virtues of the Day of Arafah
There are numerous virtues of the 9th of Dhu ’l-Hijjah which is known as yawm al-‘Arafah. This is the day where the pilgrims assemble on the plain of ‘Arafah to complete one of the essential rituals of the Hajj.
From among the many merits of this day, a few are that this is the day the religion was perfected and the blessings of Allah were complete. The following verse was revealed to the Messenger of Allah (Allah bless him and give him peace) on the day of ‘Arafah:Allah says: “Today, I have perfected your religion for you, and have completed My blessing upon you, and chosen Islam as Dīn (religion and a way of life) for you”. (Al Maidah: V3)
The completion of Allah’s blessing refers to forgiveness for ones sins by Allah, as without it the blessings of Allah cannot be complete. This brings to light the importance of being forgiven by Allah.
Fasting on Arafah day is an expiation for sins committed the year before and the year after:
حدثنا أحمد بن عبدة أنبأنا حماد بن زيد حدثنا غيلان بن جرير عن عبد الله بن معبد الزماني عن أبي قتادة قال قال رسول الله صلى الله عليه وسلم صيام يوم عرفة إني أحتسب على الله أن يكفر السنة التي قبله والتي بعده
Abu Qatadah (may Allah be pleased with him) is reported to have said that the Prophet (peace and blessings be upon him) said: “Fasting on the day of ‘Arafah is an expiation (of sins) for two years, the year preceding it and the year following it. (Reported by Ibn Majah)