[Prererularl]

This is from the amal of Madina that Imam Malik said everybody in Madina would fast on Jumuah. I think it would be good to do (and it makes sense considering the fadail of Jumuah) but for some reason you never hear anyone mention that this was from the practice of the early Muslims.

[ziIReIGS]

Friday fasting, or make a Friday as one special day to fast, is Makrooh in Shafi'i madhab.

[Andrew1978]

In the Hanafi madhhab, it's permissible to fast on Fridays but I don't know if it is a sunnah like how Mondays and Thursdays are.

[Nopayof]

It was proven in al-Saheehayn in the hadeeth of Abu Hurayrah (may Allaah be pleased with him) that he said: “I heard the Prophet (peace and blessings of Allaah be upon him) say, ‘None of you should fast on a Friday unless he fasts the day before or the day after.’” (Narrated by al-Bukhaari, 1849, Muslim, 1929). Muslim narrated in his Saheeh that the Prophet (peace and blessings of Allaah be upon him) said, “Do not single out the night of Jumu’ah [i.e., Thursday night] from among other nights for praying qiyaam al-layl, and do not single out Friday from among other days for fasting, unless it is a fast that one of you regularly observes.” (al-Siyaam, 1930)

In al-Saheeh it was narrated from Juwayriyah bint al-Haarith (may Allaah be pleased with her) that the Prophet (peace and blessings of Allaah be upon him) entered upon her on a Friday when she was fasting. He asked her, “Did you fast yesterday?” She said, “No.” He asked, “Are you going to fast tomorrow?” She said, “No.” He said, “Then break your fast.” Hammaad ibn al-Ja’d said, “I heard Qutaadah say, Abu Ayyoob told me that Juwayriyah spoke to him and he told her to stop her fast and she did so.” (Narrated by al-Bukhaari, al-Sawm, 1850).

Ibn Qudaamah said: “It is makrooh to single out the day of Friday for fasting, unless that coincides with a fast which one usually observes, such as a person who fasts alternate days, and a day that he fasts coincides with Friday, or a person who has the habit of fasting on the first or last day of the month, or the middle day of the month.” (al-Mughni, vol. 3, p. 53)

Al-Nawawi said: “Our companions (i.e. the Shaafa’is) said: it is makrooh to single out the day of Friday for fasting, but if one joins it with the day before or the day after, or it coincides with a day which he habitually fasts, or he vowed to fast the day that his sick loved one was cured, or the day So and so returns, and that day happens to be a Friday, then it is not makrooh (al-Majmoo’ Sharh al-Muhadhdhab, vol. 6, p. 479)