Not as in Rocky or Creed (which I enjoyed much more than I thought I would—thank you for “insisting” that we watch the movie together). Rather as in Boxes to Give to People.
The history of Boxing Day goes back to the Middle Ages (as in history, not as in your parents’ age) when the servants all had to work on Christmas so that the upper class (as in the people they worked for) could enjoy the Christmas Holiday without having to, um, work.
The servants were then (sometimes) given the day after Christmas off (after having worked twice as hard to be sure everything was ready for the upper class (as in the people they worked for) as well as working twice as hard the day after Boxing Day to clean up. But they were (often) given Boxing Day off.
And the tradition was that that the merchant class and upper class would take boxes (see where I’m going with this?), fill them with food and gifts, and deliver them to the servants, tradespeople, and other less fortunate types.
Now you know.