The day after Thanksgiving when the holidy/christmas shopping season starts. It is called Black Friday because it is on Friday and traditionally the day that retailers finances go in the black, i.e., become profitable. In the red, on other other hand, implies that a company is operating at a loss, i.e. unprofitable.
A sharp rise in profits from holiday sales pushed the firm in the black which is typically referred to as Black Friday in the retailing world.