Black Friday is merely the day after Thanksgiving (in the US). The day represents the kickoff of the holiday buying season. It also just so happens, that a lot of folks get the day after Thanksgiving off as it is always celebrated on last Thursday in November and gives everyone a 4 day weekend.
In addition, the name “Black Friday” arose as a result of many retailers claims that they go “black” or profitable during this time period that ends of course-Dec 31. Prior to this they were “red” or not yet covering their overhead for the year.