Q: Is it permissible to fast on Friday or Saturday? I've heard we shouldn't fast those days alone.
A: Based on the hadith reported by Imam Ahmad, "Do not fast on Saturday except for that which is an obligation", it is considered disliked to intentionally single out Saturday to fast.
The exception to this rule would be when fasting either Friday or Saturday coincides with your habitual fasting such as the Day of Arafah or the Day of Ashura etc.
If you habitually fast those days, then it is not considered disliked.
Source: Kashaf al-Qina'