Black Friday is part of America's wider shopping season including the months of November and December. And it's been getting bigger and bigger over the past decade or so, with an average annual increase of 3.5 percent since the 2008 global financial crisis.

Many retailers are now trying to beat the competition with earlier promotions, even upstaging Black Friday itself. The average in-store discount was 20 percent for the entire week before Thanksgiving, while it was 37 percent for Thanksgiving Thursday, Black Friday, and Saturday. Black Friday is still the busiest shopping day for bricks-and-mortar stores, but trends show that the general footfall is decreasing.