Black Friday started originally in the US, it is the informal name for the friday following Thankgiving day in the USA. Thanksgiving day is the fourth thursday of november. The day after Thanksgiving is the beginning of the shopping season for the holidays. Black friday got its name because the streets and stores saw black of people who wanted to purchase goods. Another theory about the origin of the term is that it is the first day of the year that retailers were out of the 'red', meaning they made a profit for the year from that point onward.
The earliest use of Black Friday as a term originated from the Factory of Management and Maintenance, where they would call the day after thanksgiving "Black Friday" because a lot of the workers called in sick to have a longer weekend.
Today Black Friday is the friday after thanksgiving, meaning, the friday after the fourth thursday of the month November. It is an unoffical shopping holiday where stores around the world lower their prices significantly.