‘Aashooraa’ is the 10th day of Muharram (the first month of the Islamic calendar). There is a great reward in fasting on the day of ‘Aashooraa’. There are many Hadeeths that mention its great virtues. It is highly recommended to fast on this day, although it is not obligatory.
Mu‘aawiyah Ibn Abi Sufyaan, may Allah be pleased with him, reported that he heard the Messenger of Allah saying: “Concerning the day of ‘Aashooraa’, it is not obligatory upon you to fast on it as I do. Whoever wishes may fast and whoever does not wish, is not obliged to do so.” [Al-Bukhari and Muslim]
Fasting the 10th day of Muharram is of great merits. The Prophet said: "Fasting the day of ‘Aashooraa’ (is of great merits), I hope that Allah will accept it as an expiation for the previous year.” [Muslim]
Ibn ‘Abbaas, may Allah be pleased with him, said: “I never saw Allah's Messenger so keen to fast any day and give it priority over any other than this day, the day of ‘Aashooraa’ and this month, meaning Ramadan.”[Al-Bukhari]
Abu Hurayrah, may Allah be pleased with him, reported: “I asked the Prophet : 'Which prayer is the best after the obligatory prayers?' He said: 'Prayer during the middle of the night.' I asked: 'Which fast is the best after the fast of Ramadan?’ He said: 'The month of Allah that you call Muharram.’” [Muslim, Ahmad and Abu Daawood]
Fasting ‘Aashooraa’ was obligatory in the beginning. In the second year of Hijrah (624 CE) when Allah's Command came that Muslims should fast the whole month of Ramadan, the Prophet sent someone to announce to people that fasting of ‘Aashooraa’ had become voluntary.
The Messenger of Allah used to fast on this day with his people in Makkah before the fasting on the month of Ramadan was prescribed. When he migrated to Al-Madeenah, he found the Jews fasting on this day. He continued fasting on this day and ordered the Muslims to fast too until it became voluntary. In another report, it is mentioned that Jews of Al-Madeenah used to hold feast on this day. The Prophet told his Companions, may Allah be pleased with them, to fast instead. [Al-Bukhari]
‘Aa’ishah, may Allah be pleased with her, stated: "The tribe of Quraysh used to fast on the day of ‘Aashooraa' in the days before Islam, as did the Prophet . When he came to Madeenah, he still fasted on it and ordered the people to do likewise. Then, when fasting during the month of Ramadan became obligatory, he.. said: 'Whoever wishes may fast [‘Aashooraa’] and whoever wishes may leave it.” [Al-Bukhari and Muslim]
When the Prophet came to Madeenah, he found that the Jews of Madeenah observing fasting on this day. The Prophet asked them the reason for their fast. They replied: “This is a blessed day. On this day, Allah saved Moosa (Moses may Allah exalt his mention) and the Children of Israel from their enemy and so Prophet Moosa fasted on this day." The Prophet said: "I am worthier of Moosa than you are.” He fasted on that day and commanded the Muslims to fast on that day. [Al-Bukhari]
Abu Moosa Al-Ash'ari, may Allah be pleased with him, reported: "The Jews would honor the day of ‘Aashooraa’ as an'Eed (feast). The Prophet said: 'You [Muslims] are to fast on it.’” [Al-Bukhari and Muslim]
One may wonder why the Prophet observed fast on the same day as the Jews while he previously ordered the Muslims not to follow non-Muslims. According to authentic Hadeeths, the Prophet used to fast on this day even before he emigrated to Madeenah. The Arabs before Islam used to fast on that day and glorified it, and it was the day when they used to cover the Ka'bah.
Later, before the end of his life, the Prophet told Muslims to add the 9th day also. If one fasts on this day, it is better to fast the 9th of Muharram or one day before ‘Aashooraa’ (i.e. the 9th) and one after it (i.e. the 11th) to differ from the Jews.
Muslim scholars one of whom was Ibn Al-Qayyim, may Allah mercy upon him, have stated that fasting of ‘Aashooraa’ is of three levels:
1. Fasting for three consecutive days, i.e., on the 9th, 10th, and 11th of Muharram. The Prophet said: “Observe fasting on the day of ‘Aashooraa’, but differ from the Jews and fast one day before it and one day after it." [Ahmad]
2. Fasting the 9th and 10th of Muharram. Ibn ‘Abbaas, may Allah be pleased with him, used to say: “We are to fast on two days: the 9th and 10th of Muharram to distinguish ourselves from the Jews.” [At-Tirmithi]
Ibn ‘Abbaas, may Allah be pleased with him, also narrated that the Prophet said: “If I live to next year, I shall also fast on the 9th day (as well as the 10th)." [Ahmad]
3. Fasting only 10th of Muharram.
This is the guidance of the Prophet regarding to ‘Aashooraa'. His Companions, may Allah be pleased with them, and their followers went on this way. The Prophet and the Companions, may Allah be pleased with them, and Muslim scholars have not distinguished this day by any practice except fasting.
But many evil acts and innovations done by some people who “deform the face of Islam” have no evidence from Quran and Sunnah. These acts include celebrating on this day, slapping one's face, tearing one's clothes, using swords and blood-shedding. The enemies of Islam use them to misrepresent Islam.