"Snopes.com, a Web site devoted to exploring urban legend, hoaxes and rumors, says Boxing Day is about preserving class lines in society. Social equals would exchange gifts on or before Christmas Day, but the less fortunate would get their gifts the day after Christmas. Those receiving gifts on Boxing Day were never supposed to give back to their benefactors, because they would be seen as uppity for thinking they were equal to those who gave them charity."

I have NOT checked this on Snopes because I have difficulty finding stuff there.