Scholarly opinions on a woman vowing to fast a certain day but then menstruated during that day
9-8-2015 | IslamWeb
Question:
I made a vow to Allah, the Exalted, to fast the three white days (i.e. the 13th, 14th and 15th) of each lunar month. However, I menstruated on those days. Should I make up for them or, given that the vow was to fast, should I not?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
Muslim scholars held different opinions about what a woman who makes a vow to fast on particular days but then experiences menstruation or post-partum bleeding on those days is required to do in this regard.
It is not incumbent on her to make up for those days or offer expiation because the vow was to fast on particular days and those days have elapsed, accordingly, the vow expires by the expiration of its specific time. This is the opinion of the Maaliki scholars, the correct opinion of the Shaafi‘i school of Fiqh, and one of the opinions reported from Imaam Ahmad 
. According to the correct opinion of the Hanbali scholars, the woman in this case is required to make up for those days and offer expiation like for breaking an oath. As for the opinion of the Hanafi scholars, the other opinion of Ash-Shaafi‘i, and the other opinion of Imaam Ahmad 
according to them, she is only required to make up for the fast-days that she vowed to fast. This is the preponderant opinion in this regard because making a vow to fast on a certain day renders this fasting obligatory. If the person is hindered from observing an obligatory fast by a Sharee'ah-considered impediment, then it is incumbent on the person to make up for the obligatory fasts that he failed to observe. Allaah, the Exalted, says (what means): {…So whoever sights [the new moon of] the month, let him fast it; and whoever is ill or on a journey—then an equal number of other days…} [Quran 2:185]
Therefore, you should fast these three days at their due time as you had vowed. If they happened to coincide with your menstruation or post-partum bleeding, you should make up for them on some other days.
Allaah Knows best.