Ashura is the tenth day of Muḥarram, the first month of the lunar calendar.  First of all, it is recommended to fast as many days as a person can do since the Prophet (peace be upon him) has been reported as saying, “The best of fasting after Ramadan is fasting Allah’s month of Muharram.” (Muslim)

At the beginning of Islam and before the prescription of fasting on Muslims, Ashura was obligatory to fast. Ibn `Abbas (may Allah be pleased with him) said, “The Prophet, (peace be upon him) came to Medina and saw the Jews fasting on the day of Ashura. He said `What is this? They said, `This is a blessed day, it is the day when Allah saved the Children of Israel from their enemies, so Moses used to fat that day.’ He said, `We have more right to follow Moses than you,’ so he fasted on that day and commanded (the Muslims) to fast on that day.” (Al-Bukhari)

Later, when the obligation of fasting Ramadan has been revealed, Muslims are recommended to fast during Ashura. Ibn Abbas (may Allah be pleased with him) said, “I never saw the Messenger of Allah so keen to fast any day and give it priority over any other time than the day of Ashura and the month of Ramadan.” (Al-Bukhari)

The Prophet said, “For fasting the day of Ashura, I hope that Allah will accept it as expiation for the past year.” (Muslim)

It is recommended to fast the ninth in addition to the tenth of Muharram. `Abdullah ibn `Abbas, may Allah be pleased with her, said, “When the Messenger of Allah fasted on Ashura and commanded the Muslims to fast as well, they said, `O Messenger of Allah, it is a day that is venerated by the Jews and Christians.’ The Messenger of Allah said : “If I live to see the next year, ‘Insha’Allah’, we will fast on the ninth day too.’ But it so happened that the Messenger of Allah  passed away before the next year came.” (Muslim)

Allah Almighty knows best.