Twentieth of Sivan explained

The Twentieth of Sivan is a historic Jewish fast day, first instituted by Rabbeinu Tam in 1171. It originally commemorated that year's massacre at Blois in France, the first blood libel in continental Europe.

The day was later also marked to commemorate the Cossack riots of 1648–49 in Poland-Lithuania, instituted by the Council of Four Lands in 1650. After World War II, suggestions were made to observe it as a Holocaust memorial day, but this was not widely adopted.^{[1] [2] [3] [4]} In 1948, Tzvi Pesach Frank proposed to use the day to commemorate the fall of the Jewish Quarter in the Battle for Jerusalem.^[5]

Notes and References

1. Web site: Aron . Yossi . Sivan 20 – The almost forgotten fast day . 2023-08-03 . www.australianjewishnews.com . en-US.
2. Web site: Mizrahi . Israel . 2021-08-19 . A Day Of Tragedies – 20 Sivan . 2023-08-03 . en-US.
3. Book: Yerushalmi, Yosef Hayim . Zakhor: Jewish History and Jewish Memory . 2011-07-01 . University of Washington Press . 978-0-295-80383-8 . 48–52 . en.
4. Book: Teller, Adam . Rescue the Surviving Souls: The Great Jewish Refugee Crisis of the Seventeenth Century . 2020-04-14 . Princeton University Press . 978-0-691-19986-3 . 54–61 . en.
5. https://twitter.com/Adderabbi/status/1667152344901906432 על חומותיך ירושלים