Actually, the Prophet (peace and blessings be upon him) has exhorted us to fast during certain days: six days of Shawwal, the first ten days of the month of Dhul-Hijjah, especially the day of `Arafah for those who are not performing Hajj, fasting Mondays and Thursdays, fasting three days (the 13th , 14th, and 15th) of every month, and fasting on Fridays as long as we fast one day before or one day after as it is reported that the Prophet ordered his followers not to single out Friday as a day of fasting unless one fasts one day before it or one day after it.
Dr. Rif`at Fawzi, former head of the department of Shari`ah at Cairo University, states the following: “If Friday corresponds with any of the the 13th , 14th, and 15th days of the lunar month, then there is nothing wrong to fast it and it is not necessary then to fast one day after it. This is an opinion attributed to the Malikis.”