The reason why it is not allowed to fast on Saturdays
Question

I have read on your site that the Prophet (peace and blessings of Allah be upon him) forbade fasting on Saturdays. What is the reason for this prohibition?.
Praise be to Allah.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.