Good King Wenceslaus and "Boxing Day"

When I was small- and growing up the child of generically Protestant parents who could accept neither the Christian Science nor the Roman Catholicism of their respective backgrounds, and had not yet stumbled upon Lutheranism- I always wondered about that "Feast of Stephen" mentioned in Good King Wenceslaus. Even then, I'd figured out that the Pogo version,

Good King Sauerkraut, look out,
On your feets uneven...

was probably inauthentic.

A biography of St. Wenceslaus, the Bohemian monarch whose piety inspired the carol, is found here.

It wasn't until much later that I realized that "the Feast of Stephen" referred to the day after Christmas, December 26, when the Church (somewhat arbitrarily) celebrates the martyrdom (and hence, the birthday into eternal life) of Stephen, Deacon and Martyr, whose story is told in Acts 6-8.

More enigmatic is another name by which this day is known, chiefly in Canada and Great Britain: "Boxing Day." I remember a conversation my sister and I had about this designation, which is generally given the day after Christmas on calendars even in the States, where we don't observe it. We decided that the most logical explanation was that, after twenty-four hours, siblings would have tired of playing with their own new toys, and begun playing with each other's- resulting in altercations which frequently came to blows.

A more probable explanation I heard once is that it is customary in those countries to put away the Christmas decorations for another year (in boxes), not on Epiphany (January 6), but on the day after Christmas. I hope, for the sake of Canadian and British kids, that this isn't true. Can anyone from Canada or Great Britain enlighten me?