The Snopes comments on Boxing Day suggest the origin is pretty vague but probably its origins are found in a long-ago practice of giving cash or durable goods to those of the lower classes. Gifts among equals were exchanged on or before Christmas Day, but beneficences to those less fortunate were bestowed the day after. Snopes also gives a number of 'origins' that are often asserted but are not true.

I'm interested in the question of when Boxing Day is. Normally, it is the 26th December but this year I am told it is not, because that is a Sunday, so Boxing Day is the day after.

Is there any traditional support for moving the day around in this fashion?