This is a day traditionally for eating leftovers and giving to the poor. Supposedly the origin is possibly about the wealthy allowing their servants to take gifts to their family. Or churches collecting “alms boxes” during Christmas week. The day after Christmas is also the feast of St. Stephen who is known for his acts of charity.
It is British tradition, so growing up, I was not aware of it. I suspect bankers were too hungover from Christmas to want to return to work, so they created Boxing Day.
I guess it came about after the United States left, so it is not something in our consciousness. Canada and other parts of the Commonwealth have it.