n. — Urban culture
the six-day period between Boxing Day (the day after Christmas), and New Year’s Eve, during which stores hold sales.
Type: 5. Frequency — The term Boxing Week refers to the entire week of sales that begin on Boxing Day and end on New Year's Eve. In the 1990s, Boxing Day sales were more generally extended from one day to an entire week (see the 1990 quotation), though some uses are found earlier. According to the Canadian Encyclopedia, the origin of Boxing Day likely comes from "the traditional opening on that day of Anglican Church poor boxes that had been filled during Advent" or from the old custom of members of the aristocracy giving servants, employees and the like gifts the day after Christmas.
The term Boxing Week is used most frequently in Canada, though it has some currency elsewhere (see Chart 1). Within Canada, the term is used most often in Ontario, followed by New Brunswick. One of the first attestations of the term in written Canadian English appears to be in the early 1970s (see the 1973 quotation). The origin of the term is likely an extension from Boxing Day.
See also COD-2, s.v. "Boxing Week", which is marked "Cdn".
Images:

Chart 1: Internet Domain Search, 1 Oct. 2013 
Chart 2: Regional Domain Search, 5 Nov. 2013