Conditions of permissibility of singling out saturday for fasting
15-5-2017 | IslamWeb
Question:
Concerning fasting the white days, the 13th, 14th, and 15th, of the Hijri month, may the Muslim fast one or two days of these days, or should he fast them all, especially if the 15th, for example, is a Friday or any other day which is disliked to be fasted by itself. Is it permissible to fast it as one of the white days?
May Allah reward you.
Answer:
All perfect praise be to Allah, the Lord of the worlds. I testify that none is worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
The Sunnah is that one fasts all three white days together. If one fasts only one or two days, he did not perform the Sunnah in the most perfect manner; however, he attains the reward for his good deed. Fasting the white days on the days that are prohibited to be singled out with fasting is permissible because the dislike is eliminated by not singling them out, as is evident.
As for fasting one day of the white days while not fasting the others or one of the days that are prohibited to be singled out with fasting, what is understood from the words of Sheikh Al-‘Uthaymeen 
is that this is not disliked if he does not intend singling out this day with fasting; rather, he fasted it for its virtue that is stated in the Sharee’ah. Sheikh Ibn Al-‘Uthaymeen summarized the cases of singling out Saturday with fasting as follows:
The first case: if it is an obligation, like fasting in Ramadan or making up for it, an expiation instead of the hadi (sacrifice) of Tamattu‘ (Hajj) and the like. There is no harm in this unless he specifies it, thinking that it has a privilege.
The second case: if he fasts on Friday along with Saturday; there is no harm in that.
The third case: that its fasting coincides with legislated days like the white days, the day of ‘Arafah, the day of ‘Ashura’, the six days of Shawwal for those who fasted in Ramadan, and the nine days of Thul-Hijjah; there is no harm because he did not fast it because it is Saturday; rather, he did so because it is one of the days that are legislated to be fasted.
The fourth case: if it coincides with a habit like the habit of the one who fasts every other day. If the day of fasting coincides with Saturday, then there is no harm in that, as the Prophet, sallallaahu ‘alayhi wa sallam, said when he prohibited fasting one or two days directly before Ramadan: “… unless it is a day on which one is in the habit of fasting (i.e. voluntary fasting that coincides with that day).” This is similar to that.
The fifth case: that he singles it out with voluntary fasting. This is the prohibited case if the hadith of prohibiting this is sound.
Allah knows best.