Black Friday, the day after Thanksgiving in the United States, is historically one of the busiest retail shopping days of the year. Many consider it the "official" beginning to the Christmas shopping season. The "black" in the name comes from the standard accounting practice of using red ink to denote negative values (i.e., losses) and black ink to denote positive values (profits). Black Friday is the day when retailers traditionally get back "in the black" after operating "in the red" for the previous months.
In Canada, Boxing Day is also observed as a public holiday, and is a day when stores sell their excess Christmas inventory at significantly reduced prices. Boxing Day has become so important for retailers that they often extend it into a "Boxing Week". This occurs similarly in Australia and New Zealand.
*both from Wikipedia
Negative, dear Nixy.