1. For the nafl fast it is permissible to specify the intention by saying: “I am keeping a nafl fast”, or to merely say: “I am keeping a fast”. Both are correct.
2. It is permissible to make the intention for a nafl fast until an hour before mid-day. If, for example, until 10am a person did not have the intention of fasting and had not consumed any food or drink until then and thereafter he decided to fast and kept the fast; this fast will also be valid.
3. Apart from the month of Ramadaan, a person may keep nafl fasts whenever he wishes. The more he fasts, the greater the reward. However, it is not permissible to fast on the days of eid ul-fitr, eid ul-ad’haa, and the 11th, 12th, and 13th of Zil Hijjah. In the entire year, it is haraam to fast on these five days only. Apart from these days, it is permissible to fast on all other days.
4. If a person takes an oath that he will fast on the day of Eid, the fast of that day will still not be permissible. In place of that day, he should keep it on some other day.
5. If a person takes an oath that he will fast for the entire year and that he will not leave out a single day, even then he cannot fast on these five days. He should fast on the remainder of the days and thereafter keep qada of these five days.
6. By making the intention of keeping a nafl fast, this nafl fast becomes wajib on the person. If a person makes an intention before subah saadiq that he is going to fast today and later (after subah saadiq) breaks this fast, it will be wajib on him to make qada of this fast.
7. A person made an intention at night that he will fast the following day. However, prior to subah saadiq he changed his intention and did not fast. Qada will not be wajib.
8. It is not permissible to keep a nafl fast without the permission of the husband. If the wife fasts without her husband’s permission and he orders her to break it, it will be permissible for her to break it. When he permits her, she must make qada of that fast.
9. A guest went to a person’s house, or, a person was invited to a meal somewhere. If the person who was invited feels that by his not partaking of the meal, his host will feel insulted or feel ill at heart, it will be permissible to break the nafl fast. It is also permissible for the host to break his nafl fast on account of the guest.
10. A person made the intention of fasting on Eid day and also kept the fast. Even then he should break his fast and qada will not be wajib on him.
11. It is mustahab to fast on the 10th of Muharram. It is mentioned in a Hadith that the one who fasts on this day will have his sins of the past year forgiven. (In addition to this, it is also mustahab to fast on the 9th or 11th. It is makruh to fast on the 10th alone.)
12. Similarly, there is also a great reward in fasting on the 9th of Zil Hijjah. By fasting on this day, the sins for the past year and the following year are forgiven. If a person fasts from the 1st till the 9th of Zil Hijjah, the rewards will be increased manifold.
13. Fasting on the 15th of Sha’baan and the six days after eid ul-fitr is more virtuous than keeping other nafl fasts.
14. If a person fasts on the 13th, 14th and 15th of every month, it is as if he has fasted for the entire year. Rasulullah sallallahu alayhi wa sallam used to fast on these days. He also used to fast on Mondays and Thursdays. If a person tries to fast on these days, he will also be greatly rewarded.