In 1171, 31 Yidden were burned in the city of Blois
France.
Rabeinu Tam was Gozer Ta'anis on כ'
סיון
He said:
" this Ta'anis is greater than Tzom Gedalya being it is like Yom Kippur."
4
After the גזרת ת"ח ות"ט
(Chmelnytsky ym"s 1648-9) The Va'ad Arba Artzos reintroduced the
Ta'anis of chof sivan
In 1946 The Gedolim in Hungary picked chof sivan as the day to commemorate the
Holocaust by fasting and Slichos.
A special Selichos for כ' סיון was
printed in Budapest.
In Eretz Yisroel Harav Chizkiyahu Mishkovsky zt"l along with other Gedolim tried during the war when the terrible information
started to become public
to
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.
Reb Moishe Feinstein zt"l (Igros Moshe Y.D.4:57)
explains
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.