It is disliked to fast on Friday alone, while it is permissible to fast on Friday if one fasts on a day before or a day after it. Also, it is permissible if one does not intend to fast on Friday alone according to the Hanbalis. However, the Malikis and the Hanafis permit singling out Friday with fasting, because Friday is a day that is just like any other day.
Hence, fasting on Friday as part of one’s fast of the six days of Shawwal is permissible if these six fast days are consecutive. If they are not consecutive, then fasting of Friday as part of the six days is still permissible, yet disliked.
In this context, Ibn Qudamah Al-Hanbali said in Al-Mughni,
It is disliked to fast on Friday alone, unless it [Friday] coincides with a day one is used to its fasting, such as when one fasts on a day and breaks the fast on the other day, and the fast day falls on Friday; and also when one is accustomed to fast on the first, middle, or last day of every month, and this day falls on Friday; and so on.
This is stated by Ahmad in a narration reported by the Hanbali jurist Al-Athram, who said,It was said to Abu `Abdullah Ahmad ibn Hanbal, “What about fasting on Friday?” He replied by quoting the hadith indicating that it is disliked to fast on Friday alone.” Then, he added, “[Fasting on Friday is disliked] except if it falls within one’s regular fast days, but to fast on it alone is disliked.” Then, he [the questioner] said, “I asked, ‘What about the man who is accustomed to fast on a day and break the fast on the other, and it happens that the day on which he breaks the fast is Thursday and the day on which he fasts is Friday and the other day he breaks the fast is Saturday, and so he fasts on Friday alone?'” He [Ibn Hanbal] replied, “Such a person’s intention is not to specifically fast on that day. Only intentional fasting on Friday is disliked.”
conversely, Abu Hanifah and Malik opined that it is not disliked to fast on Friday alone because it is a day that is just like any other day.
Below are narrations indicating that fasting Friday alone is disliked:
Narrated by Abu Hurayrah that Prophet Muhammad (peace and blessings be upon him) said, “None of you should fast on Friday, unless he fasts on either one day before it or one day after it” (Al-Bukhari and Muslim).
Muhammad ibn `Abbad said, “I asked Jabir, ‘Did the Prophet (peace and blessings be upon him) prohibit observing the fast on Friday?’ He said, ‘Yes.'” (Al-Bukhari and Muslim).
Also, Juwayriyah bint Al-Harith (may Allah be pleased with her) said that the Prophet visited her on a Friday while she was on a fast. He asked, “Did you fast yesterday?” She said, “No.” He asked, “Do you intend to fast tomorrow?” She said, “No.” He then said, “In that case, give up your fast today” (reported by Al-Bukhari).
There are also other hadiths in Sahih Al-Bukhari on this issue. After all, the Sunnah of the Prophet should indeed be followed.
This latter hadith indicates that it is disliked to single Friday out for fasting. This is because the Prophet’s prohibition was due to Juwayriyah’s nonobservance of fasting on a day before Friday or a day after it.
I also hear from Abu `Asim that what is disliked is to single Friday out for fasting and that if one joins another day with Friday in fasting, then fasting on it would not be disliked. This also brings to mind the hadiths narrated by Abu Hurairah and Juwayriyah. However, if fasting on Friday coincides with a day on which one regularly fasts, then it would not be disliked for one to fast on that day alone.
Besides, some Companions of the Prophet said that it is disliked to single the Nayrouz and Mahrajan days (two non-Islamic Persian feasts) out for fasting, because these two feasts are celebrated by disbelievers, and thus singling them out for fasting with the exclusion of other days would be an act of compliance with the disbelievers in celebrating these days. Hence, fasting on these two days is disliked, just like observing the fast on Saturdays is.
By analogical extension to this ruling, it is disliked to fast on any day of feasting celebrated by the disbelievers or on any day singled out by them for celebration.