May 4, 1886: About a dozen deaths at a labour rally in Chicago when dynamite was thrown into a police line. They retaliated by shooting at the crowd. A memorial was unveiled this week (reports from New York Times report | Chicago Tribune).

Historical information:
Chicago Public Library
Revolutionary Worker Online
Library of Congress
Northwestern University's homicide website
Chicago Historical Society
Wikipedia

Blog comment:
Laboring away at the Institute
B12 Partners Solipsism