All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If a woman vows to fast on specific days but did not fast them due to menstruation, she is required to make up these missed fast-days, because it is an obligatory fasting and she did not observe it due to a valid excuse, just like making up for the missed fast-days in Ramadan. Another opinion held that it is not obligatory on her to make up for those fast-days because these specific days have already elapsed. The first opinion is safer and more prudent in terms of clearing her liability from the obligation in this regard. Al-Mawsoo‘ah Al-Fiqhiyyah Al-Kuwaytiyyah states about the ruling on the one who vows to fast certain days but breaks his vow due to an excuse, whether or not it is obligatory on him to make up for such missed fast-days:
“The first opinion is that it is obligatory on the vow-maker to make up for those missed fast-days that he vowed to fast but did not, and no expiation is required of him. This opinion was held by the Hanafi scholars, Abu ‘Ubayd, the Maaliki scholars, and it was also the more correct opinion held by the Shaafi‘i scholars regarding the one who fails to fast during the period specified in the vow due to travel. However, some Maaliki scholars held that it is recommended, but not obligatory, to make up for such missed fast-days.
There is also an opinion held by some Shaafi‘i scholars that it is obligatory to make up for the one who fails to fast due to sickness. This opinion was deemed preponderant by Ibn Kajj, and it is also the outweighed opinion held by the Shaafi‘i scholars for the woman who fails to fast due to menstruation or post-partum bleeding. This opinion was also one of the opinions narrated on the authority of Ahmad ... They also supported their opinion by the fact that it is obligatory on a woman, who did not fast because of menstruation or post-partum bleeding, to make up for the missed fast-days because the time when her menstruation or post-partum bleeding occurred was a valid time for fasting per se, but she did not fast because of her special state (i.e., menstruation or post-partum bleeding) and therefore it is obligatory on her to make up for the missed fast-days afterward. This is similar to the case of a woman who does not fast in Ramadan due to menstruation or post-partum bleeding (i.e., it is incumbent to make up for the missed fast-days).
The second opinion is that whoever fails to fast the days specified in his vow due to an excuse, it is not incumbent on him to make up for those fast-days and no expiation is required of him either. This was the opinion held by the Maaliki scholars regarding the one who does not fast due to sickness or a woman who does not fast due to menstruation or post-partum bleeding. It is also the reliable opinion held by the Shaafi‘i scholars regarding the one who does not fast due to sickness, and also the preponderant opinion held by them regarding a woman who does not fast due to menstruation or post-partum bleeding. They supported their opinion by the fact that since fasting on the days of menstruation or post-partum bleeding was not possible to begin with given the occurrence of such states, these days should not be included in the period specified in the vow and accordingly it is not obligatory to make up for them, and the expiation is subordinate to the obligation of making up for the missed fast-days (and therefore takes the same ruling), and also because these days specified in the vow have elapsed.” [End of quote]
To be on the safe side, the sister who asked the question should make up for those fast-days that she vowed to fast, and it is not obligatory on her to fast them immediately after menstruation ends; rather, she may make up for the four consecutive days in the following week after she attains ritual purity, from Monday to Thursday.
Allah Knows best.