I have heard that black Friday is the day when people start shopping for Christmas. I have not done any research on this but, from my childhood, I use to shop for Christmas from the black Friday week.
Using this opportunity every brand started to give good discounts to users on account of black Friday deals. Thus, later on, this has become the biggest sale period of the year.
From a big giant like Amazon, Walmart, and etc to small local stores will try to attract the customer. Moreover, there is some site like couponx.com, Groupon.com, offers.com and etc which will offer the users with exclusive discounts and offers on all stores. You can see the below example how they give offers.