1-The Days of `Eid:

All scholars agree that such a fast is prohibited. It does not matter if the fast is obligatory or voluntary. `Umar testifies: “The Messenger of Allah has forbidden fasting on these two days. Concerning the `Eid of breaking the fast, it is for you to break your fast [of Ramadan]. On the `Eid of sacrifice, you should eat from what you sacrifice.” (Related by Ahmad, An-Nasa’i, At-Tirmidhi, Abu Dawud, and Ibn Majah.)

2-The Days of Tashreeq (The Three Days following `Eidul-Adha):

It is not permissible to fast during the three days following the `Eidul-Adha. Abu Hurairah reported that the Messenger of Allah, peace and blessings be upon him, sent `Abdullah ibn Hudhafah to announce at Mina: “You are not to fast these days. They are days of eating and drinking and Dhikr (remembrance of Allah).” (Related by Ahmad.) Ibn `Abbas reported that the Messenger of Allah, peace and blessings be upon him, sent a person to announce: “Do not fast on these days, as they are days of eating, drinking and rejoicing with one’s family.” (Recorded by At-Tabarani in Al-Awsat.)

The Shaf`iyyah allow fasting on the days of Tashreeq if there is some reason for the fasting – that is, if it is due to an oath, for expiation, or for making up a missed day of Ramadan. Those fasts that have no special reason behind them are not allowed, and there is no disagreement on this point. The Shaf`iyyah applied the same reasoning that they used in saying that prayers that are performed for a specific reason are allowed to be performed during the prohibited times of prayer [for example, the prayer of salutation to the mosque, and so on].

3-Singling out Friday As a Day of Fasting:

The day of Friday is a kind of weekly feast for Muslims and, therefore, it is prohibited to fast on that day. Most scholars say that this prohibition is one of dislike, not one of complete forbiddance.

If one fasts on the day before or after it, or if it is a day that one customarily fasts on (for example, the 13th, 14th, or 15th of the month), or if it is the day of `Arafah or `Ashurah (the tenth of the month of Muharram), then it is not disliked to fast on such a Friday.

`Abdullah ibn ‘Amr reported that the Messenger of Allah entered the room of Juwairiyah bintul-Harith while she was fasting on a Friday. He asked her: “Did you fast yesterday?” She answered, “No.” He said: “Do you plan to fast tomorrow?” She answered, “No.” Therefore he said: “Then break your fast.” (Related by Ahmad and an-Nasa’i with a good chain.)

`Ali counseled: “He who wants to [fast] voluntarily should fast on Thursday instead of Friday, for Friday is a day of eating, drinking, and remembrance.” (Related by Ibn Abu Shaibah with a good chain.)

In the two Sahihs (Al-Bukhari and Muslim), Jabir reported that the Prophet, peace and blessings be upon him, said: “Do not fast on Friday unless you fast on it together with the day before or the day after.” Muslim’s version states: “Do not exclusively choose the night of Friday [Thursday night in English] as a special night for performing the night prayers. Also, do not exclusively choose Friday as a day of fasting unless it occurs on a day that you regularly fast.”

4-Singling out Saturday as a day of fasting

Busr As-Salmi related from his sister As-Sama’ that the Messenger of Allah, peace and blessings be upon him, said: “Do not fast on Saturdays unless it is an obligatory fast. [You should not fast] even if you do not find anything [to eat] save some grape peelings or a branch of a tree to chew on.” (Related by Ahmad, An-Nasa’i, At-Tirmidhi, Abu Dawud, Ibn Majah, and Al-Hakim At-Tirmidhi said that what is disliked here is for a person to exclusively choose Saturday as a day of fasting, as it is the day that the Jews honor.

In contradiction with the preceding report, Umm Salamah reports: “The Prophet used to fast more often on Saturdays and Sundays than on the other days. He would say: ‘They are the feasts of the polytheists, and I love to differ from them.’ ” (Related by Ahmad, Al-Baihaqi, Al-Hakim, and Ibn Khuzaimah who declared it Authentic)

The Hanafiyyah, Shaf`iyyah, and Hanbaliyyah say it is disliked to fast on Saturday only  due to the preceding evidence. Malik differs from them, but the Hadith  stands as a proof against him.

5-Fasting on The “Day of Doubt”:

`Ammar ibn Yasir said: “Whoever fasts the ‘day of doubt has disobeyed Abul-Qasim (i.e. the Prophet].” (Related by An-Nasa’i, At-Tirmidhi, Abu Dawud, and Ibn Majah.) Most of the knowledgeable people act in accordance with this tradition. It is the opinion of Sufyan Ath-Thauri, Malik ibn Anas, `Abdullah ibn Al-Mubarak, Ash-Shafi’i, Ahmad, and Ishaq. They all hate that one fasts on a ‘day of doubt.’ Most of them believe that if one fasts on such a day and it turns out to be Ramadan, then that day still has to be made up later. If such a day occurs during one’s regular fasting period, then it is permissible to fast on such a day.

As related by “the group” Abu Hurairah reported that the Messenger of Allah said: “Do not precede Ramadan by fasting the day or two before it unless it is a day on which the person usually fasts.”

About this Hadith, At-Tirmidhi says: “The Hadith is Sound and Authentic and the scholars act in accordance with it. They dislike that a person should hasten Ramadan by fasting on the day before it. If a person usually fasts on a day and ‘the day of doubt’ occurs on that day, then there is no problem with his fasting on that day, in their opinion.”

6-Perpetual Fasting:

It is forbidden to do so because there are certain days of the year on which one is not allowed to fast. The Messenger of Allah said: “There is no [reward for] fasting for the one who perpetually fasts.” (Related by Ahmad, Al-Bukhari, and Muslim.)

If one breaks his fast during the days of `Eid and the days of Tashreeq, then his perpetual fasting would no longer be  considered disliked. In his comments on this issue, At-Tirmidhi says: “A group of scholars dislike fasting every day if it includes the `Eids [`Eidul-Fitr, `Eidul-Adha] and the days of Tashreeq. If one breaks his fast on those days, his action is no longer disliked, as he is no longer fasting the whole year.” The scholars are Malik, Ash-Shafi’i, Ahmad, and Ishaq.

The Prophet approved of Hamzah Al-Aslami’s numerous fasts when he told him: “Fast if you wish and break your fast if you wish.”

7-A Woman Cannot Offer Voluntary Fasting Unless After Gaining Her Husband’s Permission:

The Prophet, peace and blessings be upon him, forbade a woman to fast if her husband was present unless after gaining his permission. Abu Hurairah reported that the Prophet said: “A woman is not to fast [even] for one day while her husband is present except with his permission, unless it is during Ramadan.” (Recorded by Ahmad, al-Bukhari, and Muslim) Muslims scholars have interpreted this prohibition as one of forbiddance, and they allow the husband to end his wife’s fasting if she fasted without his permission and he seeks his right [to sexual intercourse] from her. This is also true, obviously, for days other than those of Ramadan in which case she does not need her husband’s permission. Similarly, if she fasted without his permission because he was not present, he has the right to end her fast when he returns.

If the husband is sick or incapable of intercourse, it is permissible for the woman to fast without his permission, for this case is similar is similar to the case where he is not present.

8-Offering Consecutive Days of Fasting without Eating (Al-Wisal):

Abu Hurairah reported that the Messenger of Allah, peace and blessings be upon him, said: “Do not perform Al-Wisal.” He repeated it thrice and the people said to him: “But you perform Al-Wisal, O Messenger of Allah!” He said: “You are not like me in that matter. I spend the night in such a state that Allah feeds me and gives me to drink. Devote yourselves to the deeds which you can perform.” (Related by Al-Bukhari and Muslim.)

The scholars say this prohibition implies that the act is disliked. Ahmad and Ishaq say that it is allowed to fast until the time of the pre-dawn meal as long as it is not a hardship on the one fasting. This opinion is based on what al-Bukhari recorded on the authority of Abu-Sa`id Al-Khudri: “The Messenger of Allah said:  “Do not make Al-Wisal. If one of you insists on making Al-Wisal, he may continue his fast [after sunset] until the time of the pre-dawn.”