Wa `alaykum as-Salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear brother, thank you for your question that shows your interest to know more about Islam, stick to its teachings, and carry out your religious duties.
As for optional fasting (siyam tatawu`), Muslims are highly recommended by the Sunnah to fast certain days, such as fasting of the day of `Arafah, the days of Tashriq (13th, 14th, and 15th of every lunar month), the day of `Ashura’ (10th of Muharram), Mondays and Thursdays, and six days of Shawwal.
If it happens that one of these days is Friday or Saturday, there is no problem in fasting it. A Muslim would fast on such a day because it is `Arafah or `Ashura’ not because it is Saturday or Friday.
It was narrated that the Prophet (peace and blessings of Allah be upon him) came to Juwayriyyah bint Al-Harith (one of the Prophet’s wives) one Friday and found her fasting. He asked her, “Did you fast yesterday?” She said, “No.” He asked, “Do you intend to fast tomorrow?” She said, “No.” He said, “Then, break your fast.” (Al-Bukhari)
Also, it was narrated that the Prophet (peace and blessings of Allah be upon him) said, “Don not single out Friday with fasting except when you fast a day before or after it.” (Al-Bukhari and Muslim)
Given the above, what is disliked—according to some scholars—for a Muslim is to single out Friday or Saturday with fasting. So, if one fasts a day before or after a Friday/Saturday, it will be Ok. If it happens that `Arafah—or any other recommended day for fasting—falls on a Friday or Saturday, it will be permissible to fast on it.
Allah Almighty knows best.