"Here is what The World Book Encyclopedia says about Boxing Day~
Boxing Day is a holiday celebrated in Britain, Australia, New
Zealand, and Canada. It falls on December 26, which is also St.
... The holiday may date from the Middle Ages (A.D. 400's-1500's), but
the exact origin is unknown. It may have begun with the lords and
ladies of England, who presented Christmas gifts in boxes to their
servants on December 26. Or it may have begun with priests, who opened
the church's alms (charity) boxes on the day after Christmas and
distributed the contents to the poor."
From: Annie's Boxing Day page
"The holiday Boxing Day may get its name from the 19th century English
custom of giving Christmas boxes containing food or money to family
servants and suppliers, the day after Christmas.
Another possibility is Boxing Day may have come from the opening of
church poor boxes that day.
The most basic understanding is that gifts, or boxes, were given to
those who were less fortunate, on the day after Christmas, while gifts
to those with equal standing were given on Christmas day."
source: Kidsturncentral.com - Boxing Day
For an excellent discussion of the several theories of the origin of
Boxing Days, see:
Snopes.com Urban Legends Reference Page: Boxing Day
"boxing day because"
"boxing day", boxes, "stephen's day"
I hope this helps.