Praise be to Allaah.
It is makrooh to single out a Saturday for fasting. Some of
them gave the reason that it is the ‘eid’ of the Jews, but it is more likely
to be because it is the day when the Jews refrain from working and they rest
on that day. If the Muslim fasts on that day, the fast may keep him from
working or slow him down, because the fast will make him thirsty and hungry,
so he will resemble the Jews in not working on that day. End quote.
Fataawa al-Shaykh Muhammad ibn Ibraaheem (may Allaah have mercy on him) (4/206).