Fasting is one of the greatest acts of worship that draw a Muslim closer to Allah and this is why it is an obligatory act of worship which is to be observed by Muslims for a whole month every year. Aside this obligatory fasting,  It is also a highly recommended act of worship to be observed by Muslims during the year, especially on certain days and occasions.

However, there are certain days in which fasting is prohibited based on certain traditions of the Prophet (peace and blessings be upon him). The Muslim jurists have differed on some details concerning these days.

Dr. Rajab Abu Mileeh states: 

The White Days do exist in every lunar month (namely, the 13th, 14th, and 15th of each Hijri month). Muslims are highly recommended to fast these three days every month. Ibn `Abbas said, “The Prophet (peace and blessings be upon him) would habitually fast the White Days, whether he was resident or on a journey.” Hafsah is also reported to have said, “There are four things the Prophet (peace and blessings be upon him) never abandoned: fasting the day of `Ashura’,  the first 10 days of Dhul-Hijjah, and the three days of every month, as well as [performing] the two rak`ahs of duha [forenoon]” (reported by Ahmad).

Furthermore, it is permissible for Muslims to fast the White Days even if they coincide Friday, Saturday and Sunday, though these are days in which fasting is disliked.

Fasting being disliked here is linked to singling out any of these days for fasting, such as when a Muslim decides to fast on Friday or Saturday or Sunday alone. Thus, if a person fasts all these three days consecutively, then it is not disliked. Besides, being disliked here applies to absolute volunteering; that is, when fasting any of these days is done for no Sunnah-based reason (e.g., the Sunnah of fasting on the day of `Arafah, on the 9th and 10th days of Muharram, etc.).

Fasting on Friday Alone

According to the Hanafis, it is acceptable that one singles out Friday for fasting. This is the opinion of Abu Hanifah and Muhammad. However, such fasting is commendable according to the Malikis, in light of a narration reported that Ibn `Abbas would persistently fast on that day.

conversely, Abu Yusuf said that fasting on Friday alone is described in a hadith as disliked unless it is coupled with another day that either precedes it or follows it. Hence, prudence calls for joining another day with it in fasting.

Ibn `Abidin said, “If both views (i.e., recommendation and a forbiddance) regarding fasting on that day are contained in the Sunnah, the last of the two narrations has  indicated forbiddance. In other words, it has abrogated the first  narration that implies recommendation, as Friday involves other acts of worship, and perhaps if one observes fasting on it, he or she would not be able to perform those acts of worship.”

According to the Malikis, the reason behind this forbiddance is the fear that this voluntary act of worship might turn to an obligation. But such a reason has become inconceivable with the death of the Prophet.

On the other hand, the Shafi`is and the Hanbalis deemed it disliked to single out Friday for fasting due to the following hadith of the Prophet: “None of you should fast on Friday, unless he [or she] fasts on either one day before it or one day after it” (Al-Bukhari and Muslim).

The Shafi`is and the Hanbalis also argue that through abstaining from fasting on Friday, a Muslim would be strong enough to perform the other acts of worship required on that day. Also, by abstaining from fasting on Friday, a Muslim would not be overrating that day as the Jews overrate Saturday. Furthermore, in this way, the optional fast on Friday (which is also a day of celebration) will not be thought of as an obligatory act of worship.

Fasting on Saturday Alone

The Muslim scholars agree that fasting on Saturday alone is disliked in light of the following hadith of the Prophet: “Do not fast on Saturday, unless it [such fasting] is part of what Allah has prescribed for you. And if you could not find anything to eat but grape skin or a piece of wood, you should chew it.” (At-Tirmidhi)

The reason for deeming it disliked to fast on Saturday is that this day is celebrated by the Jews, and thus singling it out for fasting would be an act of imitation of them, unless this day coincides with a day that a Muslim usually observes, such as the day of `Arafah or the day of `Ashura’.

Fasting on Sunday Alone

The Hanafis and the Shafi`is opine that deliberately singling out Sunday for fasting is disliked, unless this day coincides with a day a Muslim usually observes.

Ibn `Abidin’s view suggests that fasting on Saturday and Sunday together does not involve any imitation of the Jews or the Christians, because neither of them celebrate both days. Fasting on both Saturday and Sunday is thus permissible, just as fasting is permissible on both Sunday and Monday.

Moreover, the opinion of the Hanbali School of jurisprudence is that it is disliked to fast on any day celebrated by the Jews or  Christians, unless it coincides with a fasting day a Muslim usually observes.

In conclusion, the above mentioned explanation has clearly shown that the prohibition of fasting in those days only stands if some of those days are specified with fasting. Therefore, the prohibition does not apply to fasting them together as it does not apply to the fasting that is observed for a reason.