June 22
Violence erupted during a coal mine strike at Herrin, Ill. Thirty-six were killed, 21 of them non-union miners - 1922
And this:

June 22, 1922 - In what became known as the “Herrin Massacre,” a group of several hundred striking miners seized a group of strikebreakers who had been brought in by the Southern Illinois Coal Company. The strikers marched the strikebreakers to a woods near Herrin, Illinois, and opened fire, killing 19. Several strikers were held in the Williamson County jail, which has been turned into a historical museum focusing on the conflict. The St. Louis Globe-Democrat called the shootings “the most brutal and horrifying crime that has ever stained the garments of organized labor.”
Labor history found here:
http://www.unionist.com/today-in-labor-history &
http://www.workdayminnesota.org/index.php?history_9_06_22_2011