Tach Vtat memorial day

In 1171, 31 Yidden were burned in the city of Blois France.

Rabeinu Tam was Gozer Ta'anis on כ' סיון 

He said: 

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" this Ta'anis is greater than Tzom Gedalya being it is like Yom Kippur."

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After the גזרת ת"ח ות"ט  (C​hmelnytsky 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.