Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.121.

The day of Friday is a kind of weekly ‘id for Muslims and, therefore, it is prohibited to fast on that day. Most scholars say that this prohibition is one of dislike,9 not one of complete forbiddance.

If one fasts on the day before or after it, or if it is a day that one customarily fasts on (for example, the 13th, 14th, or 15th of the month), or if it is the day of ‘Arafah or ‘Ashurah, then it is not disliked to fast on such a Friday.

‘Abdullah ibn ‘Amr reported that the Messenger of Allah entered the room of Juwairiyah bint al-Harith while she was fasting on a Friday. He asked her: “Did you fast yesterday?” She answered, “No.” He said: “Do you plan to fast tomorrow?” She answered, “No.” Therefore he said: “Then break your fast.” This is related by Ahmad and an-Nasa’i with a good chain.

‘Amr al-‘Ashari reported that he heard the Messenger of Allah say: “Verily, Friday is an ‘id for you, so do not fast on it unless you fast the day before or after it.” This is related by al-Bazzar with a good chain.

‘Ali counselled: “He who wants to [fast] voluntarily should fast on Thursday instead of Friday, for Friday is a day of eating, drinking, and remembrance.” This is related by Ibn Abu Shaibah with a good chain.

In the two Sahih (those of al-Bukhari and Muslim), Jabir reported that the Prophet said: “Do not fast on Friday unless you fast on it together with the day before or the day after.” Muslim’s version states: “Do not exclusively choose the night of Friday [Thursday night in English] as a special night for performing the night prayers. Also, do not exclusively choose Friday as a day of fasting unless it occurs on a day that you regularly fast.”