Did a Google to explain it. It was also traditional that servants got the day off to celebrate Christmas with their families on Boxing Day. ... It's also when shops traditionally had big sales after Christmas in the UK (like Black Friday in the USA). The 26th December is also St. Stephen's Day Canada also celebrates it..
Around most places in the U.S. it is the day they box up all the Christmas wrappings to haul out to the curb for the trash man to take.