Q. What is the source for some fasting on Erev Shabbos Parshas Chukas? Why would there be a fasting day on Erev Shabbos when we usually avoid it?
A, Magen Avrohom (580: 9) and Mishna Berura (580: 16) quote the Tanya, that on Erev Shabbos Parshas Chukas, yechidim or some individuals accustom to fast. (Oz VeHadar Mishna Berura explains that it refers to the Arizal’s teachings (Shaar HaKavannos of Rav Chaim Vital, Drushei Tikkun Chatzos 1 and Pri Eitz Chaim, Shaar Tikkun Chatzo 3).
The reason given, is that on that day (in the year 1242), twenty (or twenty four) wagons replete of Sefarim Gemaros. Talmudic literature including many works of the Baalei Tosafos and other priceless and irreplaceable manuscripts, (it predated the printing age), were burned in Paris by agents of the Church and King Louis IX.
Magen Avrohom adds that it is “worthwhile for every Jew to cry for the burning of the Torah.” He then proceeds to tell of a customary annual fast specifically for this purpose, on Erev Shabbos Parshas Chukas.
Additionally on year 5408 (Tach) – 1648, two great European communities were destroyed.
The cause that it was instituted on the day of the week (Erev Shabbos) rather than on the day of the month, as all other days of fasting are, is because it was spiritually revealed to them that they should fast then, by using the Targum translation on “Zos Chukas Hatorah”
Moed Lekol Chai (quoted by Nitei Gavriel – Bein Hamtzarim 1: 1: 3) mentions that on that Friday, as he remembers from his younger years in Izmir, people would refrain from leaving home and would try to arrange all that was needed for Shabbos on the prior Thursday. Even in his present day, he adds. people avoid traveling from city to city. He further quotes from Kisvei Hakodesh, the tradition to awake early and recite the Kina or lamentation of “Shaali Serufa Baesh.”
Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit’a