Taanis of Erev Shabbos Chukas

The Mishna Brura and Be’er Heitiv say that there are people who fast on Erev Shabbos Chukas, because on Erev Shabbos Chukas twenty huge carton loads filled with seforim were burnt in France. Many years later, on Erev Shabbos Chukas in 5408, two whole communities were destroyed in the massacres of Tach V’Tat.

The Be’er Heitiv says the reason the fast wasn’t established on a particular day of the month, like Chof Sivan, was because of what was revealed in a dream. After the burning of the seforim, it was revealed in the dream that the day of the tragedy was connected to the parsha of Chukas. This was a decree of the Torah as it says in the beginning of Parshas Chukas זֹאת חֻקַּת הַתּוֹרָה - This is a statute of the Torah, which the Targum Unkelos translates as דא גזרת אוריתא - This is a decree of the Torah.

Because of the revelation of this dream, the fast day was decreed to take place specifically on Erev Shabbos Chukas, irrelevant of the calendar date. To this day there are some individuals that fast and have special minyanim, complete with Slichos and Vayichal.