Oh, I actually know this one! The origins of boxing day, the clue lies in "box". It was the day of gift giving. Christmas day was church and worship and boxing day, if you could afford it, you gave gifts. Usually it was the rich giving to the poor or their servants. I think the servants also got a day off after doing all the work and clean up for Christmas day.
Thank you. I was aware of this version, but when I was searching there seemed to be some differing views, and decided rather than pick either one, to inject a bit of humour instead.
:)