The twentieth of Sivan has been a traditional fast days for European Jewry from the time of the Crusades. It was first established as a public fast day by Rabbenu Tam and the Ba'alei Tosafot to commemorate the massacres of French Jewry by the crusaders. A factor in its selection was that the twentieth of Sivan can never occur on a Shabbat. It was chosen after the Chmielnicki massacres of 1648-49 (tach ve-tat תח-תט) by the Vaad Arba’ah Arazot as a public fast day to commemorate the murdered martyrs, since the slaughter had begun on that day in Nemirow. Another reason for its selection was that it was a remembrance by Polish Jewry of previous massacres. The Magen Avraham on Shulhan Arukh, O. H. 580, includes this day among the fast days that should be observed.