What is the ruling regarding fasting on Saturday other than during Ramadan?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is dislikable to single out fasting Saturdays.  The evidence for this is the following Hadith:

Narrated Abdullah Ibn Busr from his sister that the Prophet (Sallallahu Alaihi wa Sallam) said: “Do not fast on Saturdays unless the fasting (you are performing) is obligatory.  If you found nothing (to eat or drink) except the bark of a grape or a tree stick chew it”. [Ahmad and other Sunan narrators except Al Nasa’e]  The reason for this is that the Jews glorify Saturday and the Prophet (Sallallahu Alaihi wa Sallam) ordered us to avoid imitating them.  He specified singling out Saturdays to avoid being in contradiction with other evidence that permit fasting it.  Now if fasting Saturday is preceded or followed by fasting another day, then there is no dislikeability in that.  Also, if Saturday coincides with a day the person usually fasts like if the person fasts a day and breaks the fast the following day, he fasts Saturday like any other day.

Again, if Saturday coincides with Arafat day or Ashura, it should be fasted as well.  Moreover if Saturday coincides with a day of vow, like if the person says if my travelling…comes, I will fast the following day or if my sick person recovers I will fast the following day, and this day happens to be Saturday then there is no objection to fasting it.

On the other hand, there is a disagreement among the scholars regarding the soundness of the Hadith of Abdullah Ibn Busr.

Ibn Al Qayyim reported that 'Imam Malik said that that Hadith was wrong.'  Abu Dawud said: ‘This Hadith is abrogated’.  Al Nasa’e said: ‘It is a mixed up Hadith’.  Al Tirmizi said: ‘It is a good Hadith’.

Some scholars agree that the objection to fasting on Saturday regards fasting it alone with no day before it or after it.

Allah knows best.