1- It is haraam to fast on the day of Eid-ul-Fitr and Eid-ul-Adha. It is also haraam to fast with the Niyyat of first fast of Ramadan on a day about which it is not sure whether it is the last day of Sha'ban or the first of Ramadan.
2- It is haraam for a wife to keep a Mustahab fast if by so doing she would not be able to attend to her duties to her husband. And the obligatory precaution is that even if she can attend to her duties towards her husband, she should not observe a Mustahab fast without his permission.
3- It is haraam for the children to observe a Mustahab fast if it causes emotional suffering to their parents.
4- If a son observes a Mustahab fast without the permission of his father, and his father prohibits him from it during the day time, the son should break the fast if his disobedience would hurt the feeling of his father.
5- If a person knows that fasting is not harmful to him, he should fast even if his doctor advises that it is harmful. And if a person is certain or has a feeling that fasting is harmful to him, he should not fast even if the doctor advises for it, and if he fasts in these circumstances, his fast will not be valid if it turns out that the fast was actually harmful, or if it was not kept with the Niiyyat of Qurbat.
6- If a person has a strong feeling that it is harmful for him to fast, and owing to that feeling, fear is created in his mind, and if that feeling is commonly acceptable, he should not observe fast, and if he does, it will not be valid in the way described in the foregoing rule.
7- If a person who believes that fasting is not harmful to him, observes fast and realises after Maghrib that it was considerably harmful to him, he should, on the basis of obligatory precaution, give the qadha of that day.
8- Besides the fasts mentioned herein, there are other haraam fasts also, the details of which are found in relevant books.
9- It is Makrooh to fast on 'Ashura (10th of Muharram). It is also Makrooh to fast on the day about which one is not sure whether it is the day of 'Arafa or Eid-ul-Adha.