Short answer
It is better not to single out Friday for a voluntary fast: the Prophet ﷺ told us to fast a day before or after it. Many Hanafi scholars call fasting Friday alone makruh tanzihi (mildly disliked), and the Hanafi texts say the same of Saturday. The dislike goes if you add a day next to it, or if the day is Arafah, Ashura, a qada fast or part of a routine.
Fasting Friday alone as a voluntary fast is makruh tanzihi in the view of many Hanafi jurists, though Abu Hanifa and Muhammad are reported to have seen no harm in it. Singling out Saturday is also makruh tanzihi. There is no dislike when the day is joined to the day before or after, falls on a day of special virtue, or is a qada, vow or kaffarah fast.
Other madhhabs
- Maliki
- Fasting Friday alone is not disliked; Imam Malik reported that the people of knowledge he followed fasted it.
Explanation
The Prophet ﷺ said that none of you should fast on Friday unless he fasts a day before it or after it. Friday is the weekly day of gathering, prayer and remembrance, almost like a small Eid, and singling it out for fasting was discouraged. The Shafi'i and Hanbali schools hold the same.
Within the Hanafi school. Reports differ. Abu Hanifa and Muhammad are reported to have seen no harm in fasting Friday, while many later Hanafi jurists, and most Hanafi fatwa bodies today, follow the clear hadith and call singling it out makruh tanzihi. Either way, adding Thursday or Saturday removes any dislike.
Saturday. A hadith in Abu Dawud forbids fasting on Saturday except for an obligatory fast. Scholars differ on its authenticity, and Abu Dawud himself said it was abrogated. The Hanafi texts take it as a mild dislike of singling out Saturday, since it was a day the People of the Book honoured.
- Arafah or Ashura falls on a Friday: fast, no dislike.
- A qada or vowed fast on a Friday: fine.
- You fast every other day and it lands on a Friday: fine.
- Friday and Saturday together: fine.
See also fasting on Mondays, Thursdays and the white days and the Day of Arafah.
The evidence
Sahih al-Bukhari 1985
Sahih... لا يصوم أحدكم يوم الجمعة إلا يوما قبله أو بعده ...
"None of you should fast on Friday unless he fasts a day before it or after it."
Singling out Friday for a voluntary fast is discouraged.
Sunan Abi Dawud 2421
Graded sahih... لا تصوموا يوم السبت إلا فيما افترض عليكم ...
"Do not fast on Saturday except for what has been made obligatory on you."
The basis for the mild dislike of singling out Saturday, though its authenticity is debated.
Where the ruling is recorded
- al-Durr al-Mukhtar, Kitab al-Sawm, section on the kinds of fasts. Lists fasting Saturday alone among the makruh tanzihi fasts.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Sawm, section on the kinds of fasts. Discusses the reports on fasting Friday alone within the school.
- al-Majmu' (Nawawi), Kitab al-Siyam, chapter on voluntary fasting. The Shafi'i dislike of singling out Friday or Saturday.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.