If a person vows to keep a fast, it becomes wajib upon him to fulfil it. If he does not do so he will be sinning. Nazr is of two types. One is that the person takes a vow by specifying the day and date by saying: “O Allah! If a particular task of mine is fulfilled today, I will keep a fast for Your pleasure tomorrow.” Alternatively, he says: “O Allah! If a certain wish of mine is fulfilled, I will keep a fast on the coming Friday.” When taking such a vow, it is permissible to make the intention at night or the following day until an hour before mid-day. In both cases the vow will be completed. A person had taken a vow to fast on Friday. When Friday came, he merely made the following intention: “Today I am fasting”, without specifying that this is a nazr fast. Alternatively he made the intention of keeping a nafl fast. Even then, the nazr fast will be fulfilled. However, if a person keeps a qada fast on this Friday and forgets to keep the nazr fast, or, he remembers but intentionally keeps a qada fast, then in such a case the nazr fast will not be fulfilled. Instead, the qada fast will be considered and he will have to repeat the nazr fast.
The second type of nazr is that the person does not specify the day and date. He merely says: “O Allah! If a particular task of mine is fulfilled, I will keep a fast.” Alternatively, he did not mention the task but merely said that he will keep five fasts. For such a nazr, it is a prerequisite to make the intention at night. If the intention is made in the morning, the nazr will not be fulfilled and this fast will be regarded as a nafl fast.