The story
From black (“bad; ill-omened; marked by disaster”) + Friday. Friday has long been considered an unlucky day in ancient superstition. When a Friday coincided with another bad omen, such as falling on the 13th, or was marked by a disaster, it came to be called a Black Friday. In Philadelphia, police later used the term to describe the day after Thanksgiving, when heavy shopping crowds caused severe traffic congestion and chaos. Over time, public relations campaigns promoted a more positive, but inaccurate, explanation for the name. This widely circulated false etymology claimed that Black Friday” referred to the point in the year when retailers shifted from operating at a loss (“in the red”) to making a profit (“in the black”). Despite its popularity, this explanation is an urban legend and was even mistakenly included in sources like Wiktionary for several years.
Etymology from Wiktionary, CC BY-SA 4.0.