Copied the following from an Internet site:
While it wasn’t called “Black Friday” until the 1960s, and then not popularly called such until the last two decades, retailers have been trying to push people to shop the Friday after Thanksgiving since the late 19th /early 20th century. Around this time, it was very popular for various department stores, such as Macy’s and Eaton’s, to sponsor parades that would occur the day after Thanksgiving. These parades would typically be a major part of Christmas advertising campaigns by these stores. This, in turn, would ultimately result in a lot of people going shopping after the parades were over.
Over time, this melded into a commonly accepted unwritten rule among most major department stores to hold off on their major Christmas advertising pushes until after Thanksgiving; specifically, to hold off until after these parades were over.
By the 1930s, the Friday after Thanksgiving had become the official start of the Christmas shopping season among the vast majority of retailers out there. However, this tradition ultimately resulted in retailers being unhappy with the length of the Christmas shopping season on Novembers where the last Thursday was the fifth Thursday in November (Thanksgiving at that time was always on the last Thursday of November). Thus, with the strong encouragement of lobbyists for various retailers, President Roosevelt, in 1939, decided to change the official date of Thanksgiving to be on the second to last Thursday in November, in order to lengthen the Christmas shopping season as much as possible, with the hope that it would boost sales for retailers and, thus, boost the economy.