Some information I gleaned on the Internet about today's date, the 20th of Sivan:
In 1171, fifty-one Jews were burned at the stake in the French city of Blois. So Rabbeinu Tam declared a fast day on Chof Sivan. Observation of the fast gradually faded over the ensuing years as that tragedy was overshadowed by the 150 years of the Crusades.
20 Sivan was reinstituted as a fast day by certain communities, due to the tens, perhaps hundreds, of thousands of Jews, approximately one third of European Jewry, killed al kiddush Hashem in the Gezeiros Tach V'Tat (years 5408-5409, 1648-1649), the Chelminitzki massacres. Hundreds of Jewish communities in Poland and Ukraine were wiped out. There are special Selichos said and Vayichal is read.
The massacres started in Nemirov, Ukraine, on 20 Sivan when the Cossacks slaughtered the entire Jewish community, including the Rov, a great tzadik named Reb Yechiel Michel ben Reb Elazar, author of Shivrei Luchos. The Berdichiver Siddur says that is why this date was picked as a fast day and points out that 20 Sivan never falls out on Shabbos. The massacres spread out to other parts. This happened during the life of the Shach, Tosefos Yom Tov and many other great tzadikim. Yesod Veshoresh Ha'avodah says one should think about the terrible torture and suffering these people went through and all the Torah that was not learnt because they were killed.
The Shach was the first rov to institute a fast day on the day Chof Sivan in commemoration of the Gezeiros Tach V’Tat. He made this fast day only for his family and descendants. Then in 1652 the Rabbonim established the fast for the public.
When learning through a Shailas Chalom (a dream query) that these murderous times and the destruction of hundreds of shuls and battei medrash were the result of a lack of respect in Shul, Rav Yom-Tov Lipmann Heller, known as the Tosafos Yom Tov composed a special Mi She'beirach that is said in some Shuls for the people who are quiet and do not speak during davening.
Bogdan Chmielnicki, the 17th-century butcher of Jews, is still regarded as a Ukrainian national hero. His image is displayed on Ukrainian money and his monument in the center of Kiev is the focal point of the Ukrainian capital.