Question: Someone did not fast a day in Ramadaan and then made up for it on 20th of Shawwaal. However, this day coincided with a Friday. Many people said that it is impermissible to devote Friday to fasting. Do I have to repeat this fast?
Answer: It is established in the Prophetic Sunnah (acts, sayings, or approvals of the Prophet) that it is prohibited to single out Friday for a voluntary fast unless one fasts a day before or after it or if it coincides with a day that someone usually fasts. This is based on the Hadeeth narrated by Aboo Hurayrah (may Allaah be pleased with him) who said: The Messenger of Allaah (peace be upon him) said.. read more here.