I have read on your site that it is makrooh to single out Friday for fasting, because the Prophet (blessings and peace of Allaah be upon him) forbade doing that. Why is it not haraam to fast it, because the basic principle with regard to forbidding is that it means that something is haraam?.
Praise be to Allaah.
Perhaps it is because there is a concession in sharee’ah which allows fasting it with the day before. If it were haraam, it would not be allowed to fast it at all. End quote.
Fataawa al-Shaykh Muhammad Ibraaheem, 4/206.