The 12th and last month of the Islamic calendar marks the ninth day of Dhul-Hijjah and is known as the Day of Arafah. Pilgrims stand on the plain of Arafah to pray on a sacred day. On the Day of Arafah, Muslims from across the nation who cannot carry out the annual Hajj should spend the day fasting. The fast is in preparation for the following three days that are considered a festival celebrating the end of Hajj and honoring the Prophet Ibrahim's sacrifice.
According to Abu Hafsah, may Allah be pleased with him, the Prophet, upon whom be peace, stated that participating in fasting on the Day of Arafah forgives any sins for two years (the previous and upcoming year). Fasting throughout the 10th day of Muharram amends damage done by past sins.
Moreover, the wife of the Prophet, named Hafsah, may Allah be pleased with him, stated that there were four crucial things that the Messenger of Allah always paid attention to including participating in the fast on Ashura, Arafat, for a total of three days each month and by offering fajr sunnah prayers during early hours of the morning.
The words from Abu Hafsah, may Allah be pleased with him, and his wife illustrate how the tradition of fasting on the ninth of Dhul-Hijjah was a practice that the Prophet carried out throughout his life.
Some people claim that fasting isn't allowed on the day of Arafah. However, it isn't always made clear that this only applies to those who perform the Hajj. If an individual completes his Hajj, then they do not need to fast on the last day. The exception to the rule is a blessing or reward for him because of the challenges and hardships he faced while completing the pilgrimage.
Excluding individuals who complete Hajj, fasting on these special days is a great mercy.