Why do the Shî`ah practice what they do on the day of `Ashûrâ and why do Muslims fast on that day?
the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
The Shî`ah lament the unjust death of Husayn (may Allah be pleased with him) that occurred at Karbalâ on that day.
As Muslims, we also lament the death of al-Husayn (may Allah be pleased with him). We believe he was oppressively killed by his enemies. We disavow ourselves of everyone who shared in his murder, whether by hand, by speech, or by mere acceptance.
However, we follow the guidance of our Prophet (peace be upon him) as well as that of his family – `Ali and his sons al-Hasan and al-Husayn (may Allah be pleased with them all) – and do not practice any wailing or self-affliction on the day of al-Husayn’s death, nor do we abstain from what Allah has made lawful during the months in which they died.
There is no evidence from the Qur’ân ad Sunnah supporting the wailing and self-affliction that the Shî``ah practice on the 10th of Muharram. This was not the practice of the Companions. It is a very extreme innovation.
There are many authentic hadîth that refer to fasting on `Ashûrâ. It was established by the Prophet (peace be upon him) long before the events of Karbalâ. It is the Shî`ah who innovated practices on this day due to there excessive emulation of Husayn b. `Alî (may Allah be pleased with him). The significance of this day has nothing to do with the events that took place at Karbalâ.
It is Sunnah to fast on this day and either the day before it or the day after it Prophet Muhammad (peace be upon him) found the Jews fasting on this day. They informed him that they did so because Allah had saved Moses (peace be upon him) on this day. The Prophet (peace be upon him) said: “We have more rights to Moses than they have.” So he bade the Muslims to fast on this day, but told us to be different than the Jews and fast two days instead of one.
Source: Islam Today
-- Al Arabiya Digital