1. If anyone vows to keep a fast, then it is essential for him to observe it and non-observance would be a sin.
  2. The vow is of two kinds---One is specification of day and date as to say that if Allah accomplishes his desire today then he will keep a fast Tomorrow. Or to say that if his business is accomplished by Allah he will keep a fast on the day after or on Friday. In such a vow if the resolve of the fast is made in the previous night or on the very day one hour before mid-day, then it would be proper in both the cases.
  3. If one vowed to keep a fast on Friday and on Friday he only resolved to keep fast without specification whether it is the fast of the vow or a Nafl fast, even then the obligation of the vow would be discharged. But if on that Friday he resolved to keep a compensatory missed fast and forgot the vowed fast or deliberately resolved for a missed fast, then the fast of the vow would remain due and only the compensatory missed would he observed.
  4. The other kind of vow is that no day or date is specified and only vowed that if such and such business is accomplished by Allah, he will fast for one day or did not name any business and only resolved to keep five fasts, then in such vow  making of resolve in the previous night, is essential. It resolved in the morning then it would be treated as a Nafl fast and the fast of the vow would remain due.