Q. I would like to find out which days are permissible for observing Nafl (optional) fasts?
A. The best days to fast on, are Mondays and Thursdays. You can also fast on a Tuesday, Wednesday and Sunday.
You should refrain from fasting an optional fast on a Friday alone. About this, the Prophet (SAS) is reported to have said, ‘The day of Friday is your Eid, so do not fast. Except you fast before it or after it’ (Al Bazaar)
Similarly, you should not fast an optional fast on a Saturday alone. In this regard, the Prophet (SAS) said, ‘Do not fast on a Saturday except that which has been made compulsory upon you’. (Tirmizi)
And Allah knows best.
Mufti Waseem Khan