Fasting The Day Of ‘Ashura’ Along With The Day Preceding Or Succeeding it.
It is a Sunnah (Supererogatory act of worship following the example of the Prophet) to observe Sawm on the Day of ‘Ashura’ for the authentic Ahadeeth in which the Prophet (peace be upon him) stated so. The Prophet (peace be upon him) mentioned that the Jews used to observe Sawm on that day because it was the day on which Allah (may He be Exalted) rescued Musa (Moses, peace be upon him ) and his people and destroyed Pharoah and his people. our Prophet Muhammad thus, observed Sawm on the Day of ‘Ashura’ to express his gratitude to Allah, advised Muslims to do so, and directed us as well to observe Sawm on the day preceding or succeeding it. However, observing Sawm on the ninth and tenth is better than doing so on the tenth and eleventh. The latter form is also sufficient for it contradicts the practice of the Jews as well. One may also observe Sawm on all three days, i.e. the ninth, the tenth and the eleventh of Muharram as is reported in some narrations: Fast a day before(the Day of ‘Ashura’) and a day after it. But, it is Makruh (reprehensible) to observe Sawm on the Day of ‘Ashura’ only, i.e. without accompanying it by the preceding or the succeding the day. May Allah grant us success!
Source: Fatwas of Ibn Baz