At the annual meeting of the Society for Crypto Jewish Studies, at the University of Colorado in Colorado Springs CO, there was a brief discussion based on a report about New Christians who had a practice of fasting on Mondays and Thursdays.
Although some audience members assumed that this practice reflected reading the Torah on Mondays and Thursdays, this explanation does not seem particularly convincing. The most compelling literary reference in religious sources I surveyed was the Arba’ah Turim, Orah Hayyim 134, which I will mention below.
The tradition was widespread, and the specific custom as adopted by Conversos seems to be late medieval. (Jose Faur, In the Shadow of History, pp. 124-125).
The tractate Ta’anit is a source for the practice of fasting on Mondays and Thursdays in late antiquity. I am surprised that I did not find this more frequently referenced in sources I looked at briefly, but I did not do a comprehensive literary search and indeed, was not always able to check references and footnotes (most of the literature I surveyed was on the Internet or Google Books).
In Mishnah Taanit first chapter, we read:
MISHNA: If the seventeenth of Marcheshvan have passed without the rain having yet descended, private individuals commence to keep three fast-days. As soon as it becomes dark on the fast-days, however, it is allowed to eat and to drink; and on the fast-days themselves it is permitted to work, to bathe, to anoint the body, to wear shoes, and to perform the duty of cohabitation.
If the new moon of Kislev has arrived without rain having yet descended, the supreme court shall order three public and general fast-days. As soon as it becomes dark on those fast-days, however, it is lawful to eat and drink; and on the fast-days themselves it is permissible to work, to bathe, to anoint the body, to wear shoes, and to perform the duty of cohabitation.
This is the Gemara:
GEMARA: Who are meant by private individuals (in this Mishna)? Said R. Huna: "The rabbis." We have learned in a Baraitha that if private individuals commenced to keep the fast-days, they should fast on Monday, Thursday, and the following Monday; and they may interrupt their fast-days if a Monday or a Thursday fall on the day of the new moon or on such days as are mentioned in the Roll of Fasts (Megilat Ta’anit)
http://www.jewishvirtuallibrary.org/jsource/Talmud/taanit1.html (light editing)
Note that the fast-days are Mondays and Thursdays here according to a baraita (“external” text)not the Mishna text itself. The usual traditional understanding is that a baraita is a passage similar in authority, provenance and status of teacher similar to that accorded to Mishna texts, but was not included in the Mishna.
If no rains fall, the fasting becomes progressively broader and harsher: the first set of fasts is for prominent rabbis only and only in the daytime. As far as I can tell, for this and the other fasts, there is no reason given for Monday and Thursdays, at least not in this context. Quite possible fasting was instituted on these days for the same reason Torah reading was—these were the market days when people were able to gather.
In addition to fasting for rain, there is a tradition of fasting on a Monday, Thursday, and Monday” – often called BeHaB based on the Hebrew alphabet letters used to refer to these days. These are daytime fasts done shortly after Passover and Sukkot. The religious sources usually say it is an Ashkenazi custom because it the Tur Orach Hayyim 492 says it is a practice of German and French Jews.
Why fast on Mondays and Thursdays? How did Jews in late Medieval times understand this? Of all the explanations I saw, the most convincing idea from Jewish religious sources goes back to the Tur, which references fasting on Mondays and Thursdays and explains they are yeme ratzon “days of [particular] acceptance.” The context is a discussion of Tachanun (“suuplication” prayers), a series of what may be called prayers of penitence, supplication and atonement recited after the Amida. A longer form is said on Mondays and Thursdays than on other weekdays.
The reason that Mondays and Thursdays are yeme ratzon? “When Moses went up (for the second tablets) he went up on a Thursday and came down on a Monday.” –Tur Orah Hayyim 134.
Moses is traditionally said to have come down with the Second Tablets on Yom Kippur; if Moses was on the mountain 40 days, he went up to receive the Second Tablets on the day we would now call 30 Av. (there would be some discussion about the precise date—some might say it was 1 Elul—because the declaration of a new month would have been based on viewing the new moon, not a calculated date). These dates indeed reflect Talmudic deliberations in which the Torah is said to have been given on the 7th of Sivan, a Saturday. Moses came down on the 17th of Tammuz, a Thursday, only to find the Israelites worshipping the Golden Calf. The day Moses went up to Sinai to receive the Second Tablets would have been a Thursday as well, and Yom Kippur, the day he returned, would have been a Monday. (The traditional date for this was 2448 “after Creation”).
Perhaps the passage in the Tur is meant also to explain the Mishna and Gemara passages about why the Rabbis decreed fasts on Mondays and Thursdays, although this is not mentioned in the passage. Indeed, the most interesting thing, in the context of the present discussion, is that the Tur could be understood as explaining in general why Jews fast on Mondays and Thursdays.
Associate Acad. Prof. Lecturer in Islam and Judaism
Dept. of Religious Studies, University of Wyoming