Reference: http://www.ibnothaimeen.com
Fasting on a Friday is disliked, but not in the absolute sense. So fasting on a Friday is disliked for the one who intended to fast it and singled it out for fasting [only] due to the saying of the prophet (salAllaahu ‘alayhi wa sallam):
«Do not single out the Friday for fasting nor its night for standing [in prayer – i.e. Thursday night]»
As for if a person fasted coincidentally on a Friday due to it being a habitual practice of the person to fast, then there is no objection in that and the person is not at liberty. And likewise if he fasted the day before or the day after it [i.e. Friday] there is no objection in that and the person is not at liberty, and there is no dislike [Islamically, in the matter]. An example of the first case: if it was from the habit of a person to fast one day and leave a day [fasting alternative days] and coincidentally fasted on a Friday, then there is no wrong/ objection in that. Likewise, if it was the persons habit to fast the day of ‘Arafah and coincidentally the day of ‘Arafah landed on a Friday, then there is no objection to him fasting on that Friday nor any restriction upon him because indeed he did not single out this day [to fast] merely because of it being a Friday but rather due to it being the day of ‘Arafah and likewise even if this day coincided with the day of ‘Aashooraa’ and he was limited to it [i.e. to only fasting that day], then there is no objection in that [for him to fast] even though it would be better for him to fast the day before or day after the day of ‘Aashooraa’.
The second example: to fast the Thursday or Saturday along with the Friday. As for the one who fasted Friday without an eligible reason [such as it being other than a habit, or an act of sunnah that coincides with this day], then we say to him: if you wanted to fast Saturday [along with the Friday] then proceed with your fasting [on the Friday] and if you did not want to fast Saturday nor Thursday, then do not fast [on Friday] in accordance with what the messenger of Allaah (salAllaahu ‘alayhi wa sallam) ordered, and Allaah is the Expounder of Success.
Question: May the blessings of Allaah be upon you, is it permissible to fast the day of Friday on its own?
Response: The prophet (salAllaahu ‘alayhi wa sallam prohibited fasting the day of Friday on its own. One of his wives entered upon him or it was him who entered upon her and he found her fasting [on a Friday] so he asked her:
«did you fast yesterday?»
… and she replied: “no”
…he asked her:
«are you going to fast tomorrow?»
… she replied: “no”
…so he said:
«break your fast»
But if Friday coincided with the day of ‘Arafah for example, and a person then fasted it [i.e. only the Friday], then there is no harm or objection [in doing so] because this man [questioner] fasted [the Friday] because of it being the day of ‘Arafah and not because it was merely a Friday [and] if Friday coincided with the day of ‘Aashooraa’ and he fasted it then there is no objection in him to singularly fast that that day since he fasted it because it is the day of ‘Aashooraa’ and not because it is merely a Friday. For this reason the prophet (salAllaahu ‘alayhi wa sallam) said:
«do not single out the day of Friday for fasting nor its night for standing [in prayer, i.e. Thursday night]».
So the matter has been determined with regards to a person who does this, particularly with regards to [fasting] the day of Friday and [standing in prayer on] the night of Friday [i.e. Thursday night].
