Sh. Muḥammad b. Ṣāliḥ al-ʿUthaymīn رَحِمَهُ اللهُ was asked:

• If she is menstruating when ʿĀshūrāʾ comes, can she make up the fast?
• Is there a guiding principle for which optional deeds can be made up, and which ones cannot?

The Shaykh رَحِمَهُ اللهُ replied:

“Optional deeds are of two types: a type which has a cause, and a type which does not have a cause.

The one which has a cause – it elapses along with the elapsing of the cause and is not made up. For example: the prayer for “greeting the masjid” (taḥiyyat al-masjid). If a man comes and sits down, and his sitting continues for a long time, and then he wants to pray taḥiyyat al-masjid, there is no taḥiyyat al-masjid because it is a prayer which has a cause, it is tied to a cause, so when it elapses so does its legality.

Similar to that is what is apparent with the Day of ʿArafah and the Day of ʿĀshūrāʾ. If a person delays fasting the Day of ʿArafah and the Day of ʿĀshūrāʾ without an excuse, there is no doubt he does not make it up, and it will not benefit him if he did make it up, meaning: it will not benefit him in that being [a substitute for] the Day of ʿArafah and the Day of ʿĀshūrāʾ.

As for the case where it passes by a person while he is excused, such as the menstruating woman, the one experiencing post-natal bleeding, and the ill, then what is apparent is that it is not made up because this is specific to a specific day, the ruling of which elapses with the elapsing of that day.”

Source: Majmūʿ al-Fatāwá (20: 43, no. 399).