Black Friday is the informal name for the day after Thanksgiving, and is regarded as the beginning of the country's Christmas shopping season, despite shopping seeming to start earlier every year. The earliest evidence of the term in this context suggests it originated in Philadelphia, where it was used to describe the heavy, disruptive pedestrian and vehicle traffic on the day. This usage dates to at least 1961. It appeared in the Nov. 1951 journal “Factory Management and Maintenance” and referred to workers calling in sick after Thanksgiving to have a four-day weekend. But, that use did not catch on.
You can also call this day FAT, for Friday After Thanksgiving, since so much turkey was eaten yesterday. (Thanks to cousin Ali for that!)