Friday is regarded as a festive day (ʿīd) for Muslims. It has been prohibited by the Prophet (peace and blessings be upon him) to single out this day specifically for fasting. In a ḥadīth, the Messenger of Allah (peace and blessings be upon him) stated:
“Do not single out Friday night from among the nights for prayer, and do not single out Friday from among the days for fasting. However, if one of you has a regular fast that coincides with it, then there is no harm.”
(Muslim, Ṣiyām, 148)
Fasting on Friday alone, or similarly on Saturday alone, is considered makrūh (disliked). Friday is a day of festivity for the believers, and participating in the communal spirit of this blessed day entails not observing a voluntary fast. As for Saturday, it is associated with the particular religious observances of the Jews; therefore, singling it out for fasting is deemed inappropriate in order to avoid resemblance to them.
Accordingly, even in the case of a make-up (qaḍāʾ) fast, one should avoid scheduling fasts—whose timing lies within one’s discretion—so that they fall on these particular days. Nevertheless, if such a fast is observed, it remains legally valid (ṣaḥīḥ), and the individual’s obligation to fast is thereby fulfilled and discharged.