Boxing Day is traditionally either the day after Christmas (12/26) or the first work day after Christmas.  On this day, gifts are traditionally given to employees from employers or to the less fortunate.  There are many etymologies for the word, but one of the most interesting I found was:

Because the staff had to work on such an important day as Christmas by serving the master of the house and their family, they were given the following day off. As servants were kept away from their own families to work on a traditional religious holiday and were not able to celebrate Christmas Dinner, the customary benefit was to "box" up the leftover food from Christmas Day and send it away with the servants and their families. (Similarly, as the servants had the 26th off, the owners of the manor may have had to serve themselves pre-prepared, boxed food for that one day.) Hence the "boxing" of food became "Boxing Day".

So if you live in any of the Commonwealth of Nations, Happy Boxing Day!