See also: our list of 'Christmas Card Sayings and Expressions'.
The 26th of December, also called St. Stephen's Day.
Boxing Day is a public holiday that forms part of the Christmas festivities in most of the countries that were once part of the British Empire. It was originally the first working day after Christmas Day, but is now always celebrated on December 26th, regardless of which day of the week it falls.
Christmas boxes were originally literally earthenware boxes. In mediaeval England, these boxes were used by the poor (servants, apprentices etc.) to save money throughout the year. At Christmas, the boxes were broken open and the savings shared to fund Christmas festivities. This meaning of Christmas box dates back to at least the early 17th century. The boxes were known in France as tirelire and are referred to in Randle Cotgrave's A Dictionarie of the French and English tongues, 1611:
"Tirelire, a Christmas box; a box having a cleft on the lid, or in the side, for money to enter it; used in France by begging Fryers, and here by Butlers, and Prentices, etc."
In a similar tradition, which is almost as old as the above and which is the one that has stayed with us until the present day, Christmas boxes were gifts, usually money, given to tradespeople or others who have rendered some service throughout the year but who aren't normally paid directly by the donor - for example, office cleaners, milkmen etc.
So, why is Boxing Day so called? Sporting fixtures, which used regularly to include boxing, have taken place over the holiday season for centuries. The view that Boxing Day was a day for pugilism gets some support via the earliest reference to the name that I can find, which is in The Sporting Magazine, Volume 25, 1805:
On boxing-day, Dec. 26, a numerous assemblage of the holiday folk were amused by a hard fought battle, in St. Pancras-fields. This fight was one that afforded plenty of diversion to several pugilists and admirers of the art present.
Nevertheless, the link to boxing in that citation is just co-incidence and the origin of the name is the giving of 'Christmas box' gifts to tradespeople, which traditionally took place, not on Christmas Day, but on the first subsequent working day.