Assalamualaikum Warahmatullahi Wabarakatuh.
 

In the name of Allah, the Entirely Merciful, the Especially Merciful. All praises is due to Allah, Lord of the worlds - The Entirely Merciful, the Especially Merciful, Sovereign of the Day of Recompose. It is You we worship and You we ask for help. Guide us to the straight path - the path of those upon whom You have bestowed favor, not of those who have evoked (Your) anger or of those who are astray. Ameen.

Ashura holds special significance due to which Muhammad s.a.w used to fast on this day. Earlier on, it was obligatory to fast on the 10th of Muharram. However later, fasting was made obligatory in Ramadan only. As narrated by Aishah r.a.:

“The people used to fast on ‘Ashura (the tenth day of the month of Muharram) before the fasting of Ramadan was made obligatory. And on that day the Kaa’ba used to be covered with a cover. When Allah made the fasting of the month of Ramadan compulsory, Allah’s Apostle said, “Whoever wishes to fast (on the day of ‘Ashura’) may do so; and whoever wishes to leave it can do so.”(Bukhari 1592)

But why did he fast on this day? On this day, Prophet Musa a.s. performed the miracle with the help of Allah s.w.t. whereby he was able to save his people from the Pharaoh by dividing the sea into two parts creating a pathway for his people to safely cross the sea, while the Pharaoh’s army drowned. Hence, the Jews used to fast on Ashura i.e. the 10th of Muharram. The Prophet s.a.w himself fasted on this day and also ordered the Muslims to do so as he said that:

“I am closer to Moses than they." (Bukhari 3397)

The Companions observed that Jews also consider the day as special, fasting on the day. So Prophet s.a.w announced that from next year they will fast on the 9th of Muharram so as to distinguish themselves from the Jews. Unfortunately, Prophet s.a.w did not live to see the next year. Therefore, Muslims consider the 9th and 10th of Muharram, Ashura, as significant days in the Islamic calendar and observe fast on these days.