It is permissible to fast on any day except for the Festival days, when it is makrooh (abominable) to fast. However, an additional fasting only on Friday is considered to be makrooh tanzihi, which is acceptable but not desired. The Prophet (pbuh) stated : “ No one should fast on Fridays. However, if they begin fasting one day before it or they fast the next day too, they can fast on Fridays as well.” (Abu Dawud, Fasting, 50). Thus, it is permissible to perform qada fast or wajib fast like a vowing fast on Friday. Those who want to perform supererogatory fast on Fridays should also fast on the previous day or on the following day.