Q: Is it permissible to fast on Friday or Saturday? I’ve heard we shouldn’t fast those days alone.
A: In general, fasting those days, each by itself, is disliked. This is based on the hadith reported by both al-Bukhari and Muslim, “Do not fast on Friday unless you are to fast a day before it or a day after it” and what was reported by Imam Ahmad, “Do not fast on Saturday except for that which is an obligation”.
It is not considered disliked if these days are fasted with an additional day e.g. Friday and Saturday or Saturday and Sunday or when they coincide with your habitual fasts such as the Day of Arafah or the Day of Ashura etc.
Source: Sharh al-Muntaha & Kashaf al-Qina’