Boxing Day is a UK holiday celebrated the day after Christmas. It’s also celebrated in a number of countries that are still or were previously in the British Empire.
The name of day refers to boxes of Christmas gifts. Boxing Day was a day off for servants and they received ‘Christmas Boxes’ for themselves and their families from the master. Further information about the etymology of Boxing Day is on Wikipedia: https://en.wikipedia.org/wiki/Boxing_Day
Cats have a thing about boxes, don’t they — they love their cat boxes: