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 .