A long time ago, my mother made a vow to fast a month consecutively. Now, however, she cannot fulfill the vow because she suffers from hypertension and diabetes. She cannot fast two days consecutively. Is it permissible that I observe the fast instead of her? If it is impermissible, then what should I do? Please advise. May Allah reward you.

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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger. 

Whoever vows to obey Allaah is required to fulfill the vow. It was narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever vowed to obey Allaah, let him do so." [Al-Bukhari and others]

The woman in question is required to fast a month consecutively. If she cannot observe consecutiveness due to sickness, then she is permitted to break the fast during the days that she is unable to fast and then resume the fast on other days until she completes the month. She is also required to offer expiation for breaking an oath for her failure to observe consecutiveness.

This is the opinion of the Hanbali scholars . The author of Matalib Uli An-Nuha said, as he counted the incidents that do not affect the consecutiveness of fasting: "Menstruation, post-partum bleeding, insanity, unconsciousness, and expected sickness (in case of fast) because one is unable to avoid such things."

Ash-Shaafi‘i accorded with the Hanbali scholars in his old opinion on this subject as he held that breaking the fast for sickness does not affect consecutiveness. As for offering expiation for failing to observe consecutiveness, Ash-Shaafi‘i said: "If one breaks a (consecutive) fast due to an excuse, he has the option either to start a new fast without need to offer the expiation, since the vow is fulfilled as stipulated, or resume the fast of thirty days and offer expiation for not observing consecutiveness."

This is to be applied if the woman is able to fast but is unable to do so consecutively. If she is not able to fast, then she is required to feed a needy person for each day and to offer an expiation like that for a broken oath. This is the opinion of the Hanbali scholars .

The author of Matalib Uli An-Nuha said:

"Whoever vowed to fast but is unable to fulfill it shall feed a needy person for each day and offer an expiation like that for breaking an oath, out of treating a vow like an ordained duty. The expiation is due for not fulfilling the vow, and feeding is due because of the person’s inability to observe the obligation of fasting. Although the two reasons are different, both are effective; and none of them is dropped, as there is no reason to do so."

Allaah Knows best.