Question:
A lady fasted the 9th of Muharram and began her menses the day of Ashooraa, is it obligatory upon her to make up (the fast of Ashooraa) or is an expiation binding upon her?

Answer:
From that which is well know is that fasting is not obligatory upon the Muslim except the fasting of Ramadhaan, and the fasting of Ramadhaan is one of the five pillars of Islaam, because of the statement of the Prophet (صلى الله عليه وسلم):

Islaam is built upon five: The testimony that none has the right to be worshipped except Allah and that Muhammad is the Messenger of Allah, the establishment of the prayer, the giving of the Zakaat, the fasting of Ramadhaan and Al Hajj to the House of Allah. (Bukhaari)

As for fast of Ashooraa it was obligatory in the first affair (i.e. beginning of Islam), then it was abrogated by the (obligatory) fasting of Ramadhaan and fasting it became voluntary—meaning: fasting Muharram whilst fasting the 10th (Ashooraa) is more emphasized than fasting the remaining days of Muharram.

And based on this we say in answer to the question of this lady:

Indeed she fasted the 9th and her intention was to fast the 10th day, however it became impossible for her because of menses, then indeed it is hoped that the reward for fasting the day of Ashoora will be written for her because she made the intention of fasting it, even if it was prevented.

And when a person intends a righteous action and hastens towards the means of obtaining it, however it became impossible for him, then his reward is written for him due to the statement of [Allaah- The Blessed the Most High]:

{ومن يخرج من بيته مهاجراً إلى الله ورسوله

ثم يدركه الموت فقد وقع أجره على الله}

And whosoever leaves his home as an emigrant unto Allah and His Messenger, and death overtakes him, his reward is then surely incumbent upon Allah. Surah An-Nisaa Verse 100

And this lady upon whom came about that which prevented her from fasting the day of Ashooraa and it was (her) menses, then it is not legislated for her to make up the day of Ashooraa because the fasting of Ashooraa is a Sunnah and restricted to its day.

Therefore, if a preventative legislated (Islamic ruling) occurred, there is no making up for it because it is a Sunnah and it’s time has elapsed.

Source: http://www.ibnothaimeen.com/all/noor/article_2180.shtml)

Translated by: Abu-Abdir-Razzaaq (Amjad Khan)