Q: I would like to know the reason for one not to fast on a Friday alone.
A. There is a difference of opinion among the jurists regarding fast on a Friday. Some say it is permissible. Others say it will be permissible if a day before (Thursday) or a day after (Saturday) is added. Those who say that it is Makrooh to fast on only a Friday base their argument on the following:
It is narrated from Sayyidina Abu Hurayrah (Radhiyallaahu Anhu): The day of Jumu'ah is (like) the day of Eid. Do not make your day of Eid your day of fasting unless you add a day before or a day after.
Sayyidina Ali (Radhiyallaahu Anhu) says: Whoever among you wishes to keep a Nafl fast should fast on a Thursday. Do not fast on a Friday because it (Friday) is a day of eating, drinking and Zikr.
According to Imam Abu Haneefah (Rahmatullaahi Alayh) it is not Makrooh to only fast on a Friday. (Umdatul Qaari V17 P101 – 108).
Allah Ta'aala knows best.
Moulana Yusuf Laher
Checked and approved by: Mufti Siraj Desai