Question: I have a question concerning fasting on Fridays. I have read a Hadith which discourages Muslims from fasting on Fridays because it is a day of celebration. Could you explain whether this is true or not?
    Answer: By collecting and analyzing all the Ahadith on this subject, what comes to light is that this prohibition is not part of the Shari‘ah. It was a precautionary measure suggested by the Prophet (sws) to his companions. The reason is that Friday has a religious sanctity of its own. If the companions of the Prophet (sws) had started to regularly fast on this day, the later generations would have thought that this has a religious sanctity in Islam and that it was a special day to fast. This would have opened the door to bid‘ah (innovation). 
    Today, in my opinion, one can safely fast on Fridays since this never became a bid'ah after the Prophet (sws), and therefore the reason for which this precaution was given is already taken care of. Allah (swt), however, knows best. This is what Imam Malik is reported to have said in this regard: 

I have not heard a single person from among people of knowledge and learning, and those who are their followers forbid people from fasting on Fridays. And [in my opinion] fasting on this day is a good deed. I have seen certain men of erudition fast on this day and regard it as desirable. (Mua‘tta; Bab Jami‘ al-Siyam)