The term "Black Day in July" is actually the title of a Gordon Lightfoot song, included on the 1968 album "Did She Mention My Name." The song is about the 1967 Detroit riot that became one of the most destructive and deadliest riots in American history.
The 1967 Detroit riot took place over the course of five days, from July 23 to 27. On the morning of the 23rd, police executed a raid on an unlicensed and illegal bar called the Blind Pig, arresting everyone present. The arrests enraged the gathering crowd, causing a rapid spread of looting and violence. At the apex of the bloodshed federal troops were deployed, using machine guns and tanks in an effort to regain control.
The violence ceased on July 27, leaving a total of 43 people dead and more than 300 injured. After the riots, investigations revealed numerous cases of police brutality and cover-ups, the most famous of which involving three black teenagers who were allegedly killed by police after found partying at the Algiers Motel on July 26. The officers in question were charged but never convicted.