Question:

What is the ruling on fasting the day of Ashoora? Is it better to fast the day which is before or the day after it or to fast all of them together or fast the day of Ashooraa alone?

Answer:

Fasting the day of Ashooraa is a Sunnah due to what has been established in the authentic narrations of the Prophet (صلى الله عليه وسلم) in indication to that.  And it (Ashooraa) was a day which the Jews fasted because it was the day in which Allah saved Moosaa (عليه السلام) and his people and destroyed Firawn and his people; so our Prophet Muhammad (صلى الله عليه وسلم) fasted it as (an act of) gratefulness to Allah.  He (sallal-laahu-alayhi-wasallam) authorized fasting (the day of ashooraa) and legislated for us to fast a day before or after it.  To fast the 9th along with the 10th is better, and if the 10th is fasted along with the 11th then that is sufficient to oppose the Jews.  And if you fast both of them (9th & 11th) alongside the 10th then there is no problem in that due to what has been transmitted in some reports, “Fast a day before it and a day after it”.  As for fasting Ashooraa by itself, then it is disliked.  And Allaah (alone) is the Guardian and Owner of all success.

Source: http://www.binbaz.org.sa/mat/617

Translated by Abu Abdir-Razzaaq (Amjad Khan)