Boxing Day is the 26th of December, the day after Christmas Day.
[British]
Boxing Day in British English
noun
British
the first day (traditionally and strictly, the first weekday) after Christmas, observed as a holiday
Word origin
C19: from the custom of giving Christmas boxes to tradesmen and staff on this day
Boxing Day in American English
December 26, a legal holiday in most parts of the Commonwealth: so called from the former custom of giving gift boxes to employees, mail carriers, etc.