Answer
Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- Fasting on these days is only mustahabb (recommendable), not obligatory to be made up for when missed.
2- The ruling applies to a woman in her menses as the impediment in all such cases is something beyond her capacity.
3- If a person has an excuse for not fasting on all or some of these days, he/she will hopefully get the reward for his/her intention.
Answering your question, Dr. Muhammad Salama, PhD, Islamic Studies in English, Assistant Professor at the Faculty of Islamic Sciences al-Madinah International University (Mediu)), states:
Fasting on these days is only mustahabb (recommendable), not obligatory to be made up for when missed. By missing the prescribed meritorious days, one cannot get them back as the reward is restricted to fasting on these specific days.
However, if a person has an excuse for not fasting on all or some of these days, though he had the habit of fasting on, as in cases of sickness, travelling, having menses, then he/she will hopefully get the reward for his/her intention as mentioned in a hadith that reads: “When a servant of Allah falls sick or sets on a journey, he will get reward similar to that he gets for good deeds he used to do when he was healthy or at home.” (Al-Bukhari)
A woman in her menses falls under the same category as the impediment in all such cases is something beyond one’s capacity.
Allah Almighty knows best.