Ahmad relates in the Musnad, through the chain of Ibn Lahīʿah who said: Musa b. Wardān¹ narrated to us—on the authority of ʿUbayd al-Aʿraj²—who said: “My grandmother—meaning al-Ṣammāʾ—told me that she entered upon the Messenger of Allah (peace and blessings be upon him) on a Saturday while he was having lunch. He said, ‘Come and eat.’³ She replied, ‘I am fasting.’ He asked, ‘Did you fast yesterday?’ She answered, ‘No.’ He then said, ‘Eat, for fasting on Saturday is neither for you nor against you.’⁴
Although this isnād is weak, it is supported by other aḥādīth. Accordingly, his statement, ‘Do not fast Saturday,’ is to be understood as: do not intend fasting that day in particular, except in an obligatory fast. A person might intentionally fast it when no obligatory day remains for him but Saturday—such as someone who embraces Islam and only Saturday is left of the month—so he would fast that day alone.
Likewise, designating Saturday itself for an obligatory fast is not disliked, whereas designating it by itself for a voluntary fast is disliked. The dislike is removed only by adding another day to it or when it coincides with one’s established habit. In the obligatory case, what removes the dislike⁵ is simply its being an obligation, not any comparison with other days; in the voluntary case, the dislike is lifted by combining it with another day, by its conforming to a habit, and the like.
It may also be said that the exception mentioned in the ḥadīth excludes some instances⁶ of concession, while the remaining instances are excluded by other evidence.