(This is part of a Khutbah delivered a few years ago about forgiveness and fasting on the Day of Ashura'a)

When the Day of Judgment comes, we will without doubt all desire to steer clear from our sins and wish that we had done more good deeds that would be a cause for removal of our bad deeds.
Dear ones, of much interest to us these days should be the fasting of the 9th and the 10th of Muharram. Which will be today, and tomorrow, insha’allah. Rasul Allah (saw) said about the voluntary fasting on the Day of Ashura:
“It expiates the sins of the preceding year.” (Sahih Muslim)
Ibn Abbas (ra) reported:
“Rasul Allah (saw) came to al-Madinah and saw the Jews fasting on the day of Ashura. He asked them about that. They replied, ‘This is a good day, the day on which Allah rescued Baani Israa’eel from their enemy. So Musa observed fast on this day.’ Rasul Allah (saw) said, ‘We have more claim over Musa than you.’ So Rasul Allah (saw) observed fast on that day and ordered Muslims to observe fast (on that day).” (Sahih al-Bukhari, Muslim)
Ibn Abbas also reported,
“Rasul Allah (saw) fasted on the day of Ashura and ordered the people to fast on it. The people said, ‘O Messenger of Allah, it is a day that the Jews and Christians honor.’ Rasul Allah (saw) said, ‘When the following year comes – insha’allah – we shall fast on the ninth.’ The death of Rasul Allah (saw) came before the following year.” (Sahih Muslim, Abu Dawood)
So it is recommended to fast on the 10th of Muharram and the 9th along with it as well.
Ibn Rushd (Averoes) says that the scholars disagreed whether the fast is to be done on the 9th or the 10th of Muharram based on these traditions (Bidayat ul Mujtahid). And you can find recommendations to fast on both days in various books, such as al-Hindee’s “Mukhtasar al-Ahkaam al-Fiqhiyyah“, Jameel Zeno’s “The Pillars of Islam and Iman”, and Dr. Jaafar Shaikh Idris’ “The Fast.”
As for the wisdom behind this fast, the following is stated in the Mukhtasar version of Ibn ul Qayyim’s “Zaad ul-Ma`aad“:
“The day of Ashura was the day when Musa and his men got freedom from the hands of the Pharaoh and it was in the sacred memory of this great event that the Muslims observed voluntary fasting. The idea underlying this is to stress the affinity amongst the messengers of Allah and to show that religious devotion is a constant flow from one generation to another. Rasul Allah (saw) came not to abrogate all the earlier religious practices but to codify and preserve them for all times to come in ideal forms.”
Some sayings reported from the Salaf that can serve as an encouragement to do good deeds:
Taubah ibn Samat used to take account of himself and is reported to have counted the number of days in his life at the age of sixty. He found there were 21,500 days and shrieked: “What will happen to me if I meet the King with 21,500 sins? What will happen if there are 10,000 sins in each day?”
Al-Hasan al-Basri (mureed of Hazrat Ali (as)) of would say, “From the signs that one has drowned in sins, is lack of delight in fasting in the day and praying through the night.”
Alqamah Ibn Qais asked Abu Nu`aim – who was a devout worshiper – why he was hard on his body. “I want this body to be comfortable later on,” he replied.
Of Al-Ahnaf Ibn Qays it is reported that he was once told: “You are an aged elder; fasting would enfeeble you. But he replied: ‘By this I am making ready for a long journey. Obedience to Allah, Glorified is He, is easier to endure than His punishment.'”
So I remind myself as well as I remind you my brothers and sisters to resolve to show forgiveness and mercy to ourselves and to those around us, to shun harsh emotions such as anger and that which comes with it. And to do those good deeds, such as fasting, which we can do to prepare ourselves for this journey to Allah (swt).

Wa Allahu alim.