Black Friday is the friday after thankgivingday in the UnitedStates, where it is the beginning of the traditional Christmas shopping season.
The term Black Friday was first used to describe Sept. 24, 1869, when several financiers had tried to corner the gold market and the market crashed, and a depression ensued. Due to the depression it was observed as black friday.
You might think it contrarily describes the scene of hoards of shoppers trampling towards shopping malls. It's because it is when retailers' balance sheets supposedly move out of the red (red ink was used by accountants of old to indicated losses) and into the black (the retailers turning their profit).
Source courtesy - retailindustry.com