In 1171, 31 Yidden were burned in the city of Blois France.
Rabeinu Tam was Gozer Ta'anis on כ' סיון
" this Ta'anis is greater than Tzom Gedalya being it is like Yom Kippur."
After the גזרת ת"ח ות"ט (Chmelnytsky ym"s 1648-9) The Va'ad Arba Artzos reintroduced the Ta'anis of chof sivan
A special Selichos for כ' סיון was printed in Budapest.
establish a whole week of Aveilus (Shiva) and a single day of Taanis.
The Chazon Ish and Brisker Rov zt"l both opposed the idea
They held in our generation we are not big enough to declare a new fast day for Klall Yisroel.
As a result the Hungarian initiative gradually fell apart.
that the Gezeiras Hitler ym"s was about to destroy the whole
Jewish nation in the whole world.
This Gezeira is part of the original Gezeiras Galus which
Chazal had already established the fast days . We
are not big enough to add more to Takonas Chazal.
Whereas the Gezeiras תתנ"א & ת"ח ות"ט etc. were local
Gezeiros. In such cases we may be Gozer Ta'anis.
The same way if an individual goes
through ח"ו a צרה ל"ע may fast so too a Kehila may
establish a fast.