fast days - Does Judaism forbid fasting on Fridays and Saturdays? If so, where is this rule written and what is the reasoning behind it?

I was reading the deuterocanonical Book of Judith, and I came across the following:

Judith had lived as a widow in her house for three years and four months. She pitched a tent for herself on the roof of her house, put funeral clothing around her waist, and wore widow’s clothing. She fasted all the days of her widowhood except for the day before the Sabbath and the Sabbath itself, the day before the new moon, the day of the new moon, and the feasts and celebration days of the house of Israel. She was very beautiful and lovely to stare at. Her husband Manasseh left her gold and silver, male and female slaves, cattle, and fields, which she continued to oversee. And no one had a bad word to say about her, for she revered God greatly.

Being a Christian myself, I found this passage somewhat confusing because we Christians are forbidden to fast on Saturdays and Sundays (see the 66th Apostolic Canon) however we are required to fast on Wednesdays and Fridays (see the 69th Apostolic Canon)

However, I know that in Judaism, fasting is supposed to be done on Mondays and Thursdays, but is there any rule in the Jewish religion that forbids fasting on Fridays and Saturdays? If so, where is this rule written and what is the reasoning behind it?

Answer

Fasting is forbidden on Shabbat - whether on Friday night or Saturday during the day, due to the mitzvah of "Oneg Shabbat"- One is required to enjoy Shabbat. This obligation may be from the Torah, but is definitely present in Isaiah (58:13). Another proof text is Nechemia 8:9-10: when people were weeping due to their sins on Rosh Hashana, Nehemia and the Levites and Priests commanded them to eat and drink "because this day is holy to Hashem". Similarly one is forbidden to fast on all holy days including Shabbat (aside from Yom Kippur, when fasting is explicitly commanded).

Fasting is permitted by the Shulchan Aruch (288) if one is accustomed to fasting (during the day) every day and will suffer from eating, or if one has experienced a very disturbing dream that requires a fast according one's custom. Nevertheless, in the second case a person is required to do another fast to make up for fasting on Shabbat.

Fasting on Friday or Sunday is not a regular thing - voluntary fasts traditionally are not undertaken on Friday because of the holiness of Shabbat and the whiplash one would get by going from pleasure to suffering or back in one day. Taanit Esther is always pushed back to Thursday if it would fall on Sunday. Sometimes, the fast of 10 Tevet falls on Friday, and this is the only fast that is ever observed on that day by most people (Yom Kippur and Tisha b'av can't fall on Friday in our current calendar).

One thing that is quite confusing is her not fasting on the day before the New Moon. In fact, the day before the New Moon is a common time for fasting in the last few centuries (Yom Kippur Katan, little Yom Kippur). But in her days, presumably she didn't know whether the New Moon had been declared on that day, so she was probably trying to ensure that she didn't fast by mistake on the New Moon.