Steelworkers were on strike, 25,000 strong. It was a warm, sunny holiday, a mass picket line and rally took place in a field near the Republic Steel Plant, led by the Steelworkers Organizing Committee of the newly formed CIO. As the rally proceeded, police launched a sudden attack, killing 10 attendees and injuring at least 100, including women and children.
The Massacre is commemorated annually, this year on Saturday, May 21 at 1 pm, outdoors weather permitting, otherwise in the Steelworkers Hall at 11731 S Ave "O".

To learn more, check out these two videos: https://www.usw.org/video/the-memorial-day-massacre and, available at the ILHS online bookstore, http://www.illinoislaborhistory.org/shop/the-1937-memorial-day-massacre-dvd.