Boxing Day is a British tradition, observed also in commonwealth countries. Going back many centuries it was made an official holiday in 1871.
Also known as St. Stephen's Day, it was customarily a time for giving to the poor. The name comes partly from the boxes kept in British churches to collect money for the needy. On the day after Christmas Day it became a custom of the nineteenth century Victorians for tradesmen to collect their "Christmas boxes" or gifts in return for good and reliable service throughout the year on the day after Christmas.
In Australia, Canada, New Zealand and the UK, Boxing Day is the heaviest shopping day of the year. (Source The Encyclopedia of Trivia