On Adar 24, Czar Alexander I of Russia declared the Blood Libel -- the infamous accusation that Jews murdered Christian children to use their blood in the baking of matzah for Passover, for which thousands of Jews were massacred through the centuries -- to be false. Nevertheless, nearly a hundred years later the accusation was officially leveled against Mendel Beilis in Kiev.

Link: a brief history of the blood libel; the Beilis affair