Fasting on Friday Alone: Why Is It Forbidden?
Question

Is it true that Muslims are not supposed to keep nafl fast for one day. They must keep back to back at least two fasts because the Jews observe ONE day fast. Honestly, the reason seems absurd to me and I have a little doubt about his two-day nafl fast rule.
Praise be to Allah.Singling out Friday for fasting
Singling out a day for observing a nafl fast is permissible, unless it is a Friday or Saturday , or it is the day of ‘Ashura which is the tenth day of al-Muharram, in which case it is mustahabb to fast the day before or the day after as well. 
As for Sunday, Monday, Tuesday, Wednesday or Thursday, there is nothing wrong with fasting them on their own, rather it is Sunnah to fast on Mondays and Thursdays . 
Al-Bukhari (1985) and Muslim (1144) narrated that Abu Hurayrah (may Allah be pleased with him) said: I heard the Prophet (peace and blessings of Allah be upon him) say: “No one of you should fast on Friday, unless he fasts (a day) before it or after it.” 
According to a report narrated by Muslim: “do not single out the day of Friday for fasting, unless that coincides with a fast that one habitually observes.” 
Al-Nawawi (may Allah have mercy on him) said: “These ahadith appear to support the view of the majority of the companions of al-Shafi’i, which is that it is makruh to single out Friday for fasting unless that happens to coincide with a fast that one habitually observes. If he joins it to a fast on the day before or the day after, or it coincides with a day that he habitually fasts, such as if he vowed to fast on the day that a loved one was healed for the rest of his life, and that happens to be a Friday, then it is not makruh, because of these ahadith.” 
Why is it forbidden to observe fasting on Friday?
Al-Nawawi (may Allah have mercy on him) also said: The scholars said: 
“The reason why it is forbidden is that Friday is a day of du’a, dhikr and worship, such as ghusl, going to the prayer early and waiting for the prayer, listening to the khutbah and reciting a lot of dhikr after it. Allah says (in meaning): “Then when the (Jumu‘ah) Salah (prayer) is ended, you may disperse through the land, and seek the Bounty of Allah (by working), and remember Allah much.” [al-Jumu’ah 62:10] 
And there are other acts of worship to be done on this day, so it is mustahabb not to fast then, so that one will have more energy to do these duties and perform them in an energetic manner and be happy to do them and enjoy them without feeling bored or tired. This is like the pilgrim on the day of ‘Arafah in ‘Arafah: the Sunnah is for him not to fast, as stated above. 
If it is said: If that was the case, the prohibition would still apply to fasting the day before or the day after, because the reason still applies, the response to that is that he will attain reward for fasting the day before or the day after, which will make up for any shortfall in his duties on Friday that occurs because of his fasting. This is the correct view on the wisdom behind not singling out Friday for fasting.  
And it was said that the reason is that there is the fear of going to extremes in venerating Friday, so that they become infatuated with it as some people became infatuated about Saturday. But this is a weak view which is contradicted by the fact that Muslims offer Jumu’ah prayer and do other acts which are an expression of veneration of that day. 
And it was said that the reason for this prohibition is lest it be thought that it is obligatory. This is a weak view which is contradicted by the fact that it is recommended to fast on Mondays, so no attention should be paid to this unlikely notion. Similarly it is recommended to fast on the day of ‘Arafah and the day of ‘Ashura and so on. The correct view is that which we have stated above, and Allah knows best.”
Why is it disliked to fast on Saturday?
As for Saturday, it is makruh to single it out for fasting . The reason for that is that when a person fasts he becomes weak and he omits some things that he used to do, so he becomes like the Jews who do not work on Saturday . 
And Allah knows best.