What is Boxing day?

Boxing Day is the 26th December and is a national holiday in the UK and Ireland.

Boxing day is a national holiday and one to spend with family and friends, usually those not seen on Christmas Day itself. Left overs from the Christmas meal are traditionally eaten on this day.
Another activity that takes place on Boxing day is shopping for bargains! Sadly what was once a day of relaxation and family time sees the start of the sales. Sales used to start in January post-New Year but the desire to grab a bargain and for shops to off-load stock means many now start on Boxing Day.

Its origins however, are steeped in history and tradition....

In recent times the day has become synonymous with sport. Horse racing is particularly popular with meets all over the country. Many top football teams also play on Boxing Day.

In Ireland, the day is known as St. Stephen's Day or the Day of the Wren, to commemorate Saint Stephen, the first Christian martyr.
The Christmas Carol, Good King Wenceslas, is set on Boxing Day and is about a King in the Middle Ages who brings food to a poor family. ("The feast of Stephen")

In many European countries, including notably Germany, Poland, Scandinavia and the Netherlands, 26 December is celebrated as the Second Christmas Day.

Arguments abound on the origins of the name Boxing Day. All of the answers here are relevant, so maybe it is all of them?

A ‘Christmas Box’ in Britain is a name for a Christmas present.

Boxing Day was a day off for servants and when they received a ‘Christmas Box’ from the master. The servants would also go home to give ‘Christmas Boxes’ to their families.

A box to collect money for the poor was placed in Churches on Christmas day then opened the next day.

During the middle ages Great sailing ships when setting sail would have a sealed box containing money on board for good luck. If the voyage was a success, the box was given to a priest, opened at Christmas and the contents given to the poor.