Am I correct to suspect that it's "2nd day of Christmas" as it is in Germany (zweiter Weihnachtsfeiertag)? Difference being that Boxing Day is a merely public holiday but Christmas is at first a christian feast.
I believe the origin of “Boxing Day” is from the UK/England. Way back when people who worked as servants (maids/cooks/butlers) for rich people would have to work on Christmas Day (Dec 25th) “serving” their employers. These people were given the next day (Dec 26th) off to be with their own families. Their employers would “box up” the leftover food from Christmas Day and give it to their servants.
Here in Canada 🇨🇦, as a Patriated British colony we celebrate Boxing Day and it is in fact a statutory holiday, the same as Christmas Day. I believe this is common in most countries that are/were part of the British Commonwealth