Answered by Shaykh Faraz Rabbani
I have a question about the voluntary fast in the month of Rajab. Are we allowed to fast on a friday without fasting a day before or after?
Answer: Walaikum assalam,
I pray this finds you in the best of health and spirits. May Allah reward you for your keenness to turn to Him. May He accept your efforts, and place the subtle realities of sincerity and steadfastness in them.
Fasting on Fridays is not in itself disliked. Rather, as Ibn Abidin and others explain, it is only disliked if “singled out” (ifrad) for fasting— for example, if one imagines that there is particular merit in fasting Fridays. [Ibn Abidin, Radd al-Muhtar]
The wisdom behind Friday not being specifically recommended to fast is that there are many recommended acts of worship on Friday, and fasting could hold one back from them. [ibid.]
And Allah is the giver of success and facilitation.