Black Friday (Friday, November 28th this year) is the day after Thanksgiving (which is the fourth Thursday in November, for you non-US folks out there). It's the traditional start of the holiday shopping season in the US, and most retailers will have major sales starting that day and lasting through the holiday season.
Not sure entirely *why* it's called Black Friday, but "Black Friday" is also used to refer to the day the NYSE crashed in 1929.