December 26 or Boxing Day is an official holiday in the British Commonwealth, as well as some parts of Europe and Africa. It is Saint Stephen’s Day and traditionally a day when the upper classes gave boxes of food to the poor, hence the word Boxing Day, or perhaps it was from the alms boxes which were opened in the holiday season.
It is also the day of many sporting events, perhaps because people were generally on holiday and had the time to attend or participate. In the UK, there is generally a full program of rugby and football matches, similar to Thanksgiving and Christmas Day football in the United States. In Australia and South Africa, cricket matches are held on that day.
The King George VI Horsechase is always held on December 26. It is also the day in which some throwbacks to an earlier time participate in a mounted fox hunt or drag hunt, since hunting for mammals with dogs was prohibited in the Hunting Act of 2004. Several ice hockey tournaments are associated with December 26.
There is also an urban myth of job swapping on Boxing Day, made famous a M*A*S*H episode aired on December 26, but in my research, I was not able to find history or precedent for it.