Boxing Day in June

I was watching one of my favorite mystery shows on Net Flix thsi evening, a British one. It happened to be a Christmas episode centered around one family and in one scene the family receives a threat "You'll all be dead by Boxing Day." So that got me wondering, what exactly is Boxing Day?  I know it's the day after Christmas in England and Canada but what's it all about. So I googled it. Turns out it's not, as I first guessed, the day everyone boxes up unwanted gifts and returns them. It seems Boxing Day is the day the people in the grand houses gave gift boxes to tradesmen and servants. So now you know, if you didn't before.