As we know, Ashura is the tenth day of Muharram; the first month of the Islamic lunar year. The start of lunar months, besides Ramadan and Dhu’l-Hijjah for those performing hajj, is decided based on one of two criteria: a) sighting the new moon and b) the completion of the previous month. This view is found to be agreed on by all scholars based on the prophetic tradition, ‘Fast when you see it (the moon) and cease fasting when you see it, and if it is hidden or cloudy, complete the counting of Sha’ban as thirty days.’
The second principle applies only in the absence of the first, which is to complete thirty days for the month of Sha’ban. The basis for this is also mentioned in the hadith as well as by means of many other (similar) statements. It has been stated by a number of jurists that the overwhelming majority of scholars have agreed not to consider astronomy or calculations as a tool to confirm the beginning of hijri months.
So the individual should calculate when Ashura would take place if Dhu’l-Hijjah were either 29 or 30 days, and then fast these two consecutive days. Thus the fasting individual should be certain that he has fasted Ashura and would have fasted either the ninth and tenth, or the tenth and eleventh, both of which are good. For example, this year (1436h) Muharram began on the 25th of October according to most calendars. However, last month (which was the year 1435h), Dhu’l-Hijjah was 30 days. This means that the start of this Muharram was determined according to completion of 30 days of the preceding month.
‘Abdullah ibn ‘Abbas (may Allah be pleased with them both) said, “When the Messenger of Allah (peace be upon him) 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 (peace be upon him) said, ‘If I live to see the next year, God willing, we will fast on the ninth day too.’ But it so happened that the Messenger of Allah, peace be upon him, passed away before the next year came.”
The virtue of fasts during MuharramAbu Hurayrah (may Allah be pleased with him) said: "The Messenger of Allah (peace and blessings of Allah be upon him) said: 'The best of fasting after Ramadhan is fasting Allah's month of Muharram.' " [Reported by Muslim, 1982]
The phrase "Allah's month", connecting the name of the month to the name of Allah in a genitive grammatical structure, signifies the importance of the month. Al-Qari said: "The apparent meaning is all of the month of Muharram." But it was proven that the Prophet (peace and blessings of Allah be upon him) never fasted any whole month apart from Ramadhan, so this hadith is probably meant to encourage increasing one's fasting during Muharram, without meaning that one should fast for the entire month.
It was reported that the Prophet (peace and blessings of Allah be upon him) used to fast more in Sha'ban. It is likely that the virtue of Muharram was not revealed to him until the end of his life, before he was able to fast during this month. [Sharh an-Nawawi 'ala Sahih Muslim]
The virtue of fasting on AshuraIbn 'Abbas (may Allah be pleased with them both) said: "I never saw the Messenger of Allah (peace and blessings of Allah be upon him) so keen to fast any day and give it priority over any other than this day, the day of 'Ashura', and this month, meaning Ramadhan." [Reported by al-Bukhari, 1867]
The meaning of his being keen was that he intended to fast on that day in the hope of earning the reward for doing so. The Prophet (peace and blessings of Allah be upon him) said: "Fasting on the day of Ashura is an expiation for the (Minor) sins committed in the previous year" [Reported by Muslim, 1162] This is from the bounty of Allah towards us: for fasting one day He gives us expiation for the sins of a whole year. And Allah is the Owner of Great Bounty.
And Allah knows best.
► Why ASHURA is important?
► Why do Muslims Fast?
► The Beautiful Wisdom of Fasting