Boxing Day is a holiday celebrated the day after Christmas Day, thus being the second day of Christmastide. Though it originated as a holiday to give gifts to the poor, today Boxing Day is primarily known as a shopping holiday. It originated in the United Kingdom and is celebrated in a number of countries that previously formed part of the British Empire. Boxing Day is on 26 December, although the attached bank holiday or public holiday may take place either on that day or one or two days later (if necessary to ensure it falls on a weekday).
A full description of the history of Boxing Day and its modern celebration may be found on Wikipedia at https://en.wikipedia.org/wiki/Boxing_Day.