Imam Mansur Al-Buhuti (d. 1051 AH (AD 1656) said on the topic:
Optional fasts are a great virtue as per the hadith, “Every action of the son of Adam belongs to him and is one righteous deed that is written as ten and up to 700 or more.” Allah the Exalted says, “Every action of the son of Adam belongs to him except fasting. Indeed it is for me and I shall reward him for it.” This hadith is an indication of honour and high rank.
(1) It is sunnah to fast three days out of every month. The most virtuous thing to do is that he make these fasts on what is known as the white days as per the narration of Abu Dharr Al-Ghifari that the Prophet, peace and blessings of Allah be upon him, said, “When you fast three days out of every month, then do so on the 13th, 14th and 15th.” This is collected by Imam At-Tirmidhi in his Sunan and he declared it as authentic.
These are called the white days on account of the fact that on their nights the moon is full or nearly so.
(2) Optional fasts also include Monday and Thursday based upon the statement of the Prophet, peace and blessings of Allah be upon him, “Monday and Thursday are two days in which the deeds are raised up to the Lord of all creation and I love to be fasting at that time.” Collected by Imams Ahmad and An-Nisa’ii.
(3) Fasting six days of Shawwal is a noble fast as per the hadith, “Whoever fasted Ramadan and followed it up with six days from Shawwal, it is as if he had fasted the entire year.” This is collected by Imam Muslim. It is praiseworthy that he fast them consecutively starting immediately after the `Eid ul-Fitr celebration as this is quickly heading to doing good deeds.
(4) It is the sunnah to fast in the month of Muharram based upon the hadith, “The best fast after Ramadan is the month of Allah, Muharram.” This is collected by Imam Muslim. Of particular emphasis is the 10th and the 9th for fasting as the Prophet, peace and blessings of Allah be upon him, said, “If one fasts, it is good if he fasts the 9th and the 10th.” This hadith was used as evidence by Imam Ahmad who said, “If there should be some doubt upon him as to the beginning of the month, then let him fast three days in the beginning in order to be sure that he fasted the most praiseworthy days.”
The tenth is referred to as `Ashura’ and the fast on this day is an expiation for a year and it is sunnah at this time to show goodness and righteousness to the family and children.
(5) Fasting on the 9th of Dhul Hijjah is one of the optional fasts as the Prophet, peace and blessings of Allah be upon him, said, “There are no days that one could do righteous deeds on more beloved to Allah than these days.” The people said, “Messenger of Allah, does it mean that it is better than Jihad for the Sake of Allah?”
The Prophet, peace and blessings of Allah be upon him, said, “It is even better than Jihad in the Cause of Allah except in the case of a man who went out by himself with his wealth and he returned from the battle without anything.” This is collected by Imam Al-Bukhari
The most emphasised of the first ten days is the Day of `Arafah – 9th of Dhul Hijjah – for the one that is not on the Hajj. This is an expiation for two years of sins as said by the Prophet, peace and blessings of Allah be upon him, “The fast of the Day of `Arafah is one in which Allah will expiate the sins of the year before and the year after.”
He, peace and blessings of Allah be upon him, said of the fast of `Ashura’, “Indeed the fast on this day Allah reckons it to be an expiation for the sins of the year before.” This is collected by Imam Muslim. The Day of `Arafah in emphasised rank directly follows the Day of Tarwiyah, which is 8th of Dhul Hijjah, thus 9th of Dhul Hijjah is the Day of `Arafah.
(6) The most noble of the optional fasts is that one fast one day and not the next, then fast the day after and not the next and so forth. This is the fast of Dawud. This is based upon the command of the Prophet, peace and blessings of Allah be upon him, to `Abdullah ibn `Amr, “It is the best of all fasts.” This hadith is agreed upon by Imams Al-Bukhari and Muslim in their collections.
The condition for doing this is that he does not weaken the body to the degree that he cannot carry out other matters more virtuous than it, like establishing the Rights of Allah, Exalted be He, and the required rights of His Slaves. If he can carry out his obligations, then he may do this fast, but if he is weakened, then it is better for him to leave the fast of Dawud.
Disliked forms of fasting
(1) It is disliked to single out the month of Rajab alone to fast as this is reviving the affairs of the Age of Ignorance. If he does not fast a portion of Rajab but then fasts a portion, then this is not disliked and the ruling is lifted.
(2) It is also disliked to single out Friday for a fast as the Prophet, peace and blessings of Allah be upon him, said, “Do not fast Friday except that he also fasts the day before or after it.” This is collected by Imams Al-Bukhari and Muslim in their collections.
(3) Further to this, it is disliked to single out Saturday for a fast based upon the hadith of the Prophet, peace and blessings of Allah be upon him, “Do not fast Saturday except that it coincides with a fast you are already doing.” Collected by Imam Ahmad.
(4) And it is also disliked for the believer to fast on the day of Nawruz, Maharjan, any of the festivals of the unbelievers or any day that they single out for veneration.
(5) When it is the sunset of the 29th of Sha`ban, it is a day of doubt and disliked to single it out for fast when it is not cloudy or for some other reason. This is based upon the statement of the Prophet, peace and blessings of Allah be upon him, to `Ammar ibn Yasir, “Whoever fasted a day in which there is some doubt has disobeyed Abul Qasim, peace and blessings of Allah be upon him.” Collected by Imams Abu Dawud, At-Tirmidhi and authenticated by Al-Bukhari in the margins of his text.
(6) Fasting without a break between the two days or days at all is disliked. It is not disliked to fast from a portion of the day up to the evening although leaving this practice is better. This also covers fasting days in succession without suhur.
Impermissible forms of fasting
(1) It is impermissible to fast the two `Eids by Consensus as this prohibition is from a hadith in the two collections of Imams Al-Bukhari and Muslim. This prohibition means even if he is making up a compulsory fast.
(2) It is likewise impermissible to fast on the Days of Tashriq in Dhul Hijjah as per the statement of the Prophet, peace and blessings of Allah be upon him, “The Days of Tashriq are days of eating, drinking and making Remembrance of Allah.” Collected by Imam Muslim.
The only exception to this rule is when someone is making the tamattu` or qiran forms of hajj. In that case it is valid to fast the Days of Tashriq for whoever did not have the gift to hand over based upon the statement of Ibn `Umar and `A’ishah, “There is no dispensation for someone to fast on the Days of Tashriq except in the case of the one who did not find a gift to hand over on his hajj.” This is collected by Imam Al-Bukhari
 This is collected by Imam Al-Bukhari in Al-Jami` us-Sahih.
 Trans note: this refers to the 11th, 12th and 13th days of Dhul Hijjah
 Trans note: cf. Ar-Rawd ul-Murbi` bi-Sharhi Zad il-Mustaqni`, pp. 197-99