What is the Twentieth of Sivan?

Question: What is the Twentieth of Sivan? 

Answer: The Twentieth of Sivan is a historic Jewish fast day established by Rabbeinu Tam in 1171 to commemorate the Blois massacre in France, the first blood libel in continental Europe. It later symbolized other anti-Jewish violence during the Crusades, such as the Rhineland massacres, and was marked for the 1648–49 Cossack riots in Poland-Lithuania. Although proposed as a Holocaust memorial day post-World War II, this was not widely adopted. Some communities, like Belz, Skver, and Papa Chasidim, still recite Selichot on this day.