The ninth day of Dhul-Hijjah (the 12th and final month of the Islamic calendar) is the Day of Arafah. It is the day when pilgrims stand on the plain of Arafah to pray. On this day, Muslims all over the world who do not witness the annual Hajj should spend the day in fasting, in preparation for the three days festivity following Eid ul-Adha (the celebration marking the end of the Hajj commemorating the Prophet Ibrahims willingness of sacrifice).
Abu Hafsah, may Allah be pleased with him, reported that the Prophet, upon whom be peace, said:
(Fasting on the Day of Arafah absolves the sins for two years: the previous year and the coming year, and fasting on Ashura, (the tenth day of Muharram) atones for the sins of previous years.) [Reported by all except al-Bukhari and at-Tirmidhi]