1648-1650—The Chmielnicki Massacres and Their Aftermath

In the mid-17th century, Bogdan Chmielnicki led a Cossack and peasant uprising in Ukraine. His followers destroyed countless Jewish communities across Ukraine, seeking to eliminate all Jews from it. The remaining Jews of Europe sought to remember the massacres through a fast day, but they didn't know which day to choose. Ultimately, they decided to subsume the massacres under the 20th of Sivan, the day that the Jews of Blois were murdered. Thousands of Jews fasted on this day, every year, into the 20th century.