It occured to me a few years back that most of my students had no idea why the day after Thanksgiving is called "Black Friday". So I gave them this explanation, because it is a part of our society's culture. It makes sense.
Traditionally, that day is the day most retailers hope to be brought out of the red into the black. The red and black refer to the color ink that used to be used in accounting records. Red was in debt. Black was showing a profit.
Most small retailers have such high overhead costs that if they can hang in there til the Holiday season, they've got it made. The day after Thanksgiving is usually the day when gift-shopping begins in earnest and most stores will cater accordingly. They open earlier and close later than usual. They usually have incentitives to draw crowds to their stores, such as first 50 customers receive 50% off. Also they tend to have lots of extra stock on hand.
So, it's the spending habits that enable stores to show a profit that brings a store out of the red into the black that gives that day it's moniker. Some of my students thought it had to do with race/ethnicity. Uhm, how so? I asked. Some thought it was because that's when the Stock Market crashed. Every year. Uhm, nope.