Black Friday is the Friday following Thanksgiving and is traditionally known as the start of the holiday season.

The term "Black Friday" is believed to have originated in Philadelphia, used to describe heavy and disruptive pedestrian and vehicle traffic that would occur on the day after Thanksgiving.

Cyber Monday started as a marketing term for the Monday immediately following Black Friday. 

The idea; people return to work, and use the internet to get deals they may not have gotten on Black Friday.  A number of online dealers now offer Cyber Monday deals.