What is the reason for the prohibition of fasting on Fridays in isolation?

In a Nutshell: it is not advisable to fast on Fridays in isolation as the Prophet Muhammad (peace be upon him) forbade fasting on a particular day of the week in isolation, unless it is a day on which a person has already made a habit of fasting. Friday is a holy day and it is recommended to live it by performing acts of charity and devotion, such as the Friday prayer. Therefore, fasting on Fridays in isolation could go against the tradition and recommendation of the Prophet Muhammad (pbuh). It is permissible to reserve fasting for a particular day with the exception of Friday and Saturday or Ashura (10th day of the first month). Outside of these days, it is recommended to fast one day before or after the day one is particularly targeting. As for Sunday, Monday, Tuesday, Wednesday and Thursday, there is no harm in fasting on these days alone. On the contrary, the Sunnah recommends fasting on Thursday and Monday. Bukhari (1985) and Muslim (1144) reported that Abu Hurairah (radi allahu an ) heard the Messenger of Allah (pbuh) say: "Let none of you fast on Friday without fasting a day before or after." he version of Muslim states, "Do not reserve fasting for the day of Friday, unless it is part of a regular fast." An-Nawawi (May Allah grant him His mercy) said: "These hadith clearly support the opinion of the majority of Shafii's companions and those who agree with them that it is frowned upon to fast on Fridays in isolation; one must add a day before or after, unless one is accustomed to doing so as if one has formed a vow to fast on the day in which a sick person of the person concerned has been cured and that day coincides with Friday" In this case, the reprobation disappears, taking into account the above-mentioned hadiths. According to the ulama, the reason for the prohibition is that Friday is a day of invocation, remembrance (of Allah) and worship, including taking the ritual bath, leaving in good time to wait at the mosque for the celebration of the prayer, listening to the sermon, and intensifying the remembrance after the prayer, in application of the word of the Most High: "Then when the prayer is ended, disperse yourselves over the earth, and seek [some effect] of Allah's grace, and call upon Allah much." Qur'an, 62:10 ) and other religious acts scheduled for that day. This is why it is recommended not to fast, so that one is better able to carry out these activities with vigour, eagerness and pleasure, far from any feeling of weariness. This is similar to the state of the pilgrim on the day of Arafah, as the Sunnah recommends that he does not fast for the same reasons. If it is retorted that fasting a day before or after Friday does not solve anything, we reply that the merit that comes from fasting the day before or the day after can make up for the torpor and weakness that might mar a fasting person's performance of the Friday activities. This is the explanation given for the prohibition of fasting on Fridays in isolation. It is also said that it is for fear of exaggerating the importance of Friday as others have done with Saturday. This explanation is weak and contradicted by the celebration of the Friday prayer and other well-known acts that are part of the recommended activities to mark the importance of Friday. It is also said that the reason for the prohibition of fasting is to prevent it from being considered obligatory. This explanation is weak and contradicted by the recommendation to fast on Mondays regardless of this unlikely consideration. This is also the case for the days of Arafah, Ashura and others. The correct opinion is the one we have said is retained. Allah knows best. Conclusion You have seen how the determination of the reason for the prohibition has been the subject of interpretative efforts by the ulama. You have also seen how an-Nawawi (radi allahu an) has ruled many of the opinions that have been issued in the direction of justifying the prohibition of fasting on Fridays in isolation to be wrong. This is why it is appropriate for the Muslim to submit to the religious law, whether he understands its justifications and motives or not, because the word of the Messenger of Allah (PBUH) must be accepted with submission in accordance with the word of the Most High: "It is not for a believer or a believer's wife, once Allah and His Messenger have decided on a matter, to have any choice in their course of action. And whoever disobeys Allah and His Messenger, he has certainly gone astray from a clear error. (Qur'an,33:36) May Allah assist you and us to do what He loves and pleases.