Vowing to Fast a Particular Day

1. If a person took a vow saying: “I am going to fast on Friday,” or “I am going to fast from the 1^st of Muharram till the 10^th,” then it is not waajib to keep the fasts on those particular days.
2. Whenever one wishes, one may keep the fasts of the vow mentioned in the above case.
3. However, the 10 fasts vowed to be kept in Muharram will have to be kept in succession, whether in Muharram or any other month.
4. Similarly, if a person said in a vow: “If my work gets done today then I will fast tomorrow,” the person has a choice of fasting the next day or any other day.

(Behshti Zewar)