There is much dispute over the true origins of Boxing Day, but one common story of the holiday's origins is that servants used to receive Christmas gifts from their employers on December 26, after the family celebrations. These were generally called their Christmas boxes. Another story is that this is the day that priests broke open the collection boxes and distributed the money to the poor.
Boxing Day is now a public holiday in the UK, and traditionally a day of sport. Like other public holidays which may occur on a non-working day, the "day off" will occur on the first day after the public holiday that would otherwise be a working day. Boxing Day itself remains on the 26th if that is a Saturday, but moves to the Monday if the 26th is a Sunday (the Sunday in question being referred to as Christmas Sunday).
In Canada, Boxing Day is also observed as a public holiday, and is a day when stores, especially electronics stores, sell their excess Christmas inventory at radically reduced prices. Boxing Day has become so important for retailers that they often extend it into a Boxing Week.
In Australia, test match (cricket) starting on December 26 is called the Boxing Day Test Match, and is played at the Melbourne Cricket Ground before the largest, rowdiest crowd of the summer. In Sydney, the annual Sydney-Hobart yacht race, one of the biggest and most prestigious ocean racing events in the world, begins on this day, as the yachts depart Sydney Harbour before many thousands of spectators around the harbour and in spectator boats.