The origin of this fast has always been a mystery. A fast on the 13th of Adar is not mentioned in the Book of Esther. Nor is such a fast mentioned in Tannaitic or Amoraic literature.

Interestingly, the 13th of Adar (Yom Nikanor) is one of the holidays included in Megillat Taanit. This means that in Tannaitic times it was a day that one was not allowed to fast. (On the 13th of Adar in 161 B.C.E., Judah Maccabee defeated the Syrian military commander Nikanor, leading to the establishment of this holiday.)

We do know that by the ninth century some Jewish communities in Babylonia fasted on the 13th of Adar. We know this because a responsum of R. Natronai, head of the academy at Sura from 857-865, refers to the fast. The fast is also referred to in another source from Babylonia, the siddur of R. Saadiah (10th cent.)

In around this same period, there are four sources that refer to a practice in Eretz Yisrael of fasting three days in Adar, on a Monday-Thursday-Monday cycle. These sources are: Soferim (Chapters 17 and 21), and three other sources that have come to light from the Cairo Genizah. This custom in Eretz Yisrael surely arose to commemorate Esther having fasted three days in Nisan (Esther 4:16). But this Palestinian custom is not a fast on the 13th of Adar.

So how did the Jewish community in Babylonia come to adopt a fast on the 13th of Adar, a fast for which there seems to have been no precedent?

A widespread view is that this fast arose to commemorate the three days of fasting initiated by Esther in Nisan. There are Rishonim who take this approach. But Geonic Babylonia is where the fast on the 13th of Adar first arose and this approach is not expressed in any of the sources from Geonic Babylonia that mention the fast of the 13th of Adar.

Fasting on the 13th of Adar is referred to in one of the four she’iltot for Purim included in the she’iltot of R. Achai Gaon. This is a work composed in eighth-century Babylonia. But according to the most recent scholarship, the four she’iltot for Purim were probably not in the original she’iltot when it left the hands of R. Achai in the eighth century. They were authored in a later period. (See R. Brody, Le-Toledot Nusach ha-She’iltot, p. 186, n. 5.)

I believe that I was able to solve the mystery of the origin of the fast of the 13th of Adar. My article with the solution was published in AJS Review, Vol. 34 (November 2010), pp. 309-351. I can only allude to the solution here and I refer all to the original article or to the shorter summary that appeared in my “Esther Unmasked: Solving Eleven Mysteries of the Jewish Holidays and Liturgy” (2015).

Briefly, the solution is as follows:

The earliest source that refers to a practice of fasting on the 13th of Adar is an anonymous Babylonian Geonic responsum that made its way into Midrash Tanchuma (Bereishit, section 3). It is reasonable to work with the assumption that this responsum dates from the eighth or ninth centuries.

This responsum adopts a very unusual interpretation of the sections of the Mishnah at the beginning of tractate Megillah. These sections permit villagers to fulfill their Megillah obligation on the 11th, 12th or 13th of Adar, on yom ha-kenisah, under certain conditions. In the plain sense of these sections, yom ha-kenisah refers to Mondays and Thursdays, and the teaching is that the reading for the villagers is allowed to be advanced to these days when the villagers enter, or gather in, the cities.

But in the interpretation adopted by this Geonic responsum, yom ha-kenisah means the fast of the 13th of Adar (= the day on which the Jews gather to fast). The reading for the villagers is allowed to be advanced because the date of the observance of the fast day is being advanced due to a prohibition to fast on Shabbat and Erev Shabbat that is being read into the Mishnah. In this interpretation, the advanced fast day is a day upon which the reading for the villagers is allowed.

Close reading of the responsum suggests that the responsum is not describing a practice of fasting on the 13th that was occurring in its time. It is only interpreting Mishnah Megillah 1:1-2 and describing a practice of fasting on the 13th that theoretically occurred in ancient times, according to the interpretations it was offering.

The critical question in determining the origin of the fast of the 13th of Adar is what motivated this unusual Geonic interpretation. Obviously, one possible motivation was an attempt to justify an existing practice to fast on the 13th. But in my article I suggest something entirely different that motivated this unusual interpretation. (The motivation is too hard to explain in the short space here.) Then we can understand the practice of fasting on the 13th as having originated as a consequence of this unusual Geonic interpretation of the Mishnah at Megillah 1:1-2.

That the fast of the 13th of Adar did not arise as commemoration of the three days of fasting initiated by Esther is seen from the name for the fast day of the 13th in the earliest sources. The responsum of R. Natronai is the earliest source that refers to the fast by a name, and it refers to the fast as “Taanit Purim. R. Saadiah refers to the fast as “the fast of the Megillah.”

To sum up, the practice of fasting on the 13th of Adar originated in Geonic Babylonia. The approach most consistent with the Geonic sources is that the fast arose as a consequence of an unusual Geonic interpretation of Mishnah Megillah 1:1-2. There had not been a practice of fasting on the 13th at the time the Geonic interpretation of Mishnah Megillah 1:1-2 originated. The origin of this fast in Babylonia had nothing to do with the three days of fasting initiated by Esther in Nisan, even though today the fast bears her name.

Mitchell First can be reached at [email protected]. He is happy that it is the Babylonian practice of fasting one day (the practice not based on Esther’s actions) that prevailed, and not the more difficult Monday-Thursday-Monday practice!