c 1477 BCE- G-d brought a "very severe plague" against the people of Israel as a result of their grumbling against the LORD and their lust for meat to eat (Numbers 11:31-34).
1171- The 20th of Sivan is the anniversary of the first blood libel in France. On this date in 1171, tens of Jewish men and women were burned alive in the French town of Blois on the infamous accusation that Jews used to the blood of Christian children in the preparation of matzot for Passover.
1648- 6,000 Jews of Niemirov murdered during the Khmelnytsky massacres.
June 10, 1917- The American Jewish Congress was formed. 330,000 people participated in its first election.
June 28, 1967- Although Yom Yerushalayim is on the 28th of Iyar, 20 Sivan marks the day that Israel annexed the Old City and officially united all of Jerusalem.