It is makrooh to single out Saturday for fasting, because of the report narrated by al-Tirmidhi (744), Abu Dawood (2421) and Ibn Maajah (1726) from ‘Abd-Allaah ibn Busr, from his sister, that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Do not fast on Saturdays apart from days when you are obliged to fast. If any one of you cannot find anything other than grape stalks or the twigs of a tree, let him chew it (to make sure that he is not fasting).”Classed as saheeh by al-Albaani in al-Irwa’ (960). Abu ‘Eesa al-Tirmidhi said: This is a hasan hadeeth. What is makrooh in this case is for a man to single out Saturday for fasting, because the Jews venerate Saturday. End quote.
Ibn Qudaamah (may Allaah have mercy on him) said in al-Mughni (3/52): Our companions said: It is makrooh to single out Saturday for fasting … what is makrooh is singling out that day. If he fasts another day along with it, then it is not makrooh, because of the hadeeth of Abu Hurayrah and Juwayriyah. If it coincides with a day that a person regularly fasts, it is not makrooh.
What is meant by the hadeeth of Abu Hurayrah is the report narrated by al-Bukhaari (1985) and Muslim (1144) from Abu Hurayrah (may Allaah be pleased with him) who said: I heard the Prophet (peace and blessings of Allaah be upon him) say: “No one of you should fast on Friday, unless he fasts (a day) before it or after it.”
The hadeeth of Juwayriyah was narrated by al-Bukhaari (1986) from Juwayriyah bint al-Haarith (may Allaah be pleased with her), that the Prophet (peace and blessings of Allaah be upon him) entered upon her on a Friday and she was fasting. He said; “Did you fast yesterday?” She said, “No.” He said: “Do you intend to fast tomorrow?” She said: “No.” He said: “Then break your fast.”
This hadeeth and the one before it clearly indicate that it is permissible to fast on a Saturday at times other than Ramadaan, for the one who fasts the Friday before.
It is proven in al-Saheehayn that the Prophet (peace and blessings of Allaah be upon him) said: “The most beloved of fasting to Allaah is the fast of Dawood; he used to fast one day and not the next.”
In this case it is bound to happen that he will sometimes fast on a Saturday on its own. So it may be understood from this that if Saturday coincides with a day that he habitually fasts, such as ‘Arafah or ‘Ashoora’, there is nothing wrong with fasting on that day, even if it is on its own.
Al-Haafiz stated in al-Fath that an exception to the prohibition on fasting on Friday is made if a person habitually fasts on a certain day, such as ‘Arafah, and it coincides with Friday.
The same applies to Saturday. We have quoted the words of Ibn Qudaamah concerning that above.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said:
It should be noted that different scenarios may apply with regard to fasting on a Saturday.
1 – It may be obligatory, such as observing a Ramadaan fast during Ramadaan or making it up later on, or observing a fast as expiation (kafaarah), or instead of offering a sacrifice when performing Hajj tamattu’, and so on. There is nothing wrong with that so long as he does not single it out deliberately thinking that there is some virtue in that.
2 – If he fasts on the Friday before, there is nothing wrong with it, because the Prophet (peace and blessings of Allaah be upon him) said to one of the Mothers of the Believers who was fasting on a Friday: “Did you fast yesterday?” She said: “No.” He said: “Are you going to fast tomorrow?” She said, “No.” He said: “Then break your fast.” The words, “Are you going to fast tomorrow?” indicate that it is permissible to fast Saturday along with Friday.
3 – If it happens to be one of the days when it is prescribed to fast, such as Ayyaam al-Beed (the 13th, 14th and 15th of each hijri month), ‘Arafah, ‘Ashoora’, six days of Shawwaal for one who has fasted Ramadaan, and the ninth of Dhu’l-Hijjah. There is nothing wrong with that, because he is not fasting because it is Saturday, rather it is because it is one of the days when it is prescribed to fast.
4 – When it happens to be a day when he habitually fasts, such as a person whose habit is to fast alternate days, and the day that he fasts happens to be a Saturday, then there is nothing wrong with it, as the Prophet (peace and blessings of Allaah be upon him) said when he forbade fasting one or two days before Ramadaan begins: “except a man who (habitually) observes a fast, in which case let him fast.” This is similar.
5 – If he singles it out to observe a voluntary fast for one day only. This is what is forbidden, if the hadeeth forbidding it is proven to be saheeh.
End quote from Majmoo’ Fataawa wa Rasaa’il al-Shaykh Ibn ‘Uthaymeen (20/57)
And Allaah knows best.