It's something to do with the bonus given to employees at christmas in the olden days - read a bit of Charles Dickens that should answer it............
picture this.....Christmas Day, lots of stuff, therefore lots of stuff left over.........put it in boxes, give it to the tradesmen next day........early form of recycling?
They used to have what was known as 'Poor Boxes' in churches. The money was collected throughout the year, counted on Christmas Day and the next day (St Stephen's Day) given out to the poor and needy of the parish by the priest.
It refers to the fighting that goes on when the incredibly expensive gift you spent weeks looking for is in fact, the wrong size/wrong colour/wrong brand /tarty/tashy and the receipt can't be found.......