Today (26th December) is known as Boxing Day, and dates back to the Middle Ages at least. The reason for this name is pretty much lost in time, but there are several theories:
1. Boxing Day is the same day as the Feast of St Stephen, on which metal boxes were put outside churches to collect money for the poor.
2. In Britain, it was a custom for tradesmen to collect “Christmas boxes” of money or presents on the first weekday after Christmas as thanks for good service throughout the year. This is mentioned in Samuel Pepys’ diary entry for 19 December 1663.
3. Since they would have to wait on their masters on Christmas Day, the servants of the wealthy were allowed the next day to visit their families. Their employers would give each servant a box to take home containing gifts and bonuses, and sometimes leftover food 😉

Boxing Day shoppers crowding a store.

Boxing Day shoppers crowding a store.

In the UK, Australia, Canada and some other ex-colonial countries, Boxing Day is an important day for sports including Rugby, ice-hockey, horseback hunting, football and cricket.
Boxing Day in these countries is also a major shopping day, with sales opening often very early in the morning and shoppers camping out in queues outside their doors, in order to claim massive reductions on some products.

Thanks to Wikipedia for details.