By Rabbi Yehiel E. Poupko
«For German Jews, the Shabbat before Shavuot is called "The Black Shabbat." On this day, there is a practice amongst German Jews to recite Av Harachamim (Merciful Father). This prayer was composed by the Jews of Ashkenaz in the wake of the first Crusade in 1096, which devastated the magisterial communities of Mainz, Speyer, and Worms.
In this prayer Jewish communities ask God to hold the nations of the world accountable for the violent shedding of Jewish blood. Among its closing lines is the verse: "For the One who pursues the reckoning of violently shed blood will remember them. He has not forgotten the tortured cry of the innocent." This "Black Shabbat" always precedes Shavuot.»
JUF News : The Black Shabbat