What is the ruling on fasting Saturday voluntarily, and is it permissible to fast Saturday along with Friday, given the difference in opinions regarding the prohibition of fasting it?

The correct view among scholars is that it is reprehensible to single out Saturday for fasting, unless it coincides with a habitual fast, or is a fast to make up for a missed one (qada'), or a vow, or something similar. This is based on the Prophet's (peace be upon him) saying: "Do not fast on Saturday, unless it is for what has been made obligatory upon you." The meaning of it being reprehensible to single it out is for a man to specifically choose that day for fasting because the Jews venerate it.