Boxing Week is, at least, the six-day period that starts with Boxing Day and ends with New Year's Eve. It may also include a few days before Christmas and/or after New Year's Day. It is not strictly a week. It is a term that was invented by the retail industry a few years ago (at least in Canada) to enable them to extend (hopefully) the Boxing Day sales frenzy.
This results in huge lineups at retailers.
Many products have a mail-in rebate to be used, a tactic used by manufacturers to clear their inventory.