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 Dhul-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 Shaban. 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.
If the month of Muharram were not to be confirmed according to the criteria set out in the hadith, then the best choice a person has is to fast either 2 or 3 days in which the Ashura fasting should logically be achieved. So the individual should calculate when Ashura would take place if Dhul-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 (1433h) Muharram began on the 26th of November according to most calendars. However, in the year 1432h Dhul-Hijjah was 29 days. This means that the start of this Muharram was not confirmed according to one of the two sharii criteria mentioned earlier and hence the start of Muharram has no definitive confirmation.
The 10th day of Muharram (Ashura) will be on Monday 5th December 2011 if it happened that Dhul-Hijjah was 29 days. However, if Dhul-Hijjah was 30 days then Ashura would be Tuesday 6th December. Based on what I have stated, Ashura will definitively be on either Monday or Tuesday and consequently fasting these two days would mean that a person would have fasted the day of Ashura. Imam Ahmad ibn Hanbal stated that a person should fast three days if he doubts the beginning of the month to be sure that he has fasted the correct day. In fact al-Nawawi indicated that a possible reason as why the Prophet (peace be upon him) fasted the 9th of Muharram (as well as the 10th) was to ensure that he fasted on Ashura (and did not miss it).
Abdullah ibn Abbas (may Allah be pleased with them both) said,
| When the Messenger of Allah ﷺ 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 ﷺ 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, ﷺ, passed away before the next year came.
If an individual also wants to be sure of fasting the 9th of Muharram, then he should fast the two days we have spoken about and the day immediately before them as well. Then he would have fasted the 9th, 10th and 11th, or the 8th, 9th and 10th. In either case he definitely would have fasted the 9thand 10th.
If someone finds it extremely difficult to fast three consecutive days, then he should opt to fast the two days. If it is difficult to fast two days for whatever reason, despite the fact that days are very short in Europe this Ashura, he should calculate Ashura according to the aforementioned criteria. In another words, if he knows and has accepted that the moon of Muharram was sighted then he should calculate the day of Ashura accordingly, otherwise he should calculate it based on the second criterion that completes Dhul-Hijjah as thirty days. It would be incorrect to calculate the day of Ashura according to the calendar.
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 Al-Bukhari and Muslim, narrated by Abu Hurairah.