I always thought it was along these lines ?
Why is 26 December called Boxing Day?
Traditionally, 26 December was the day to open the Christmas Box to share the contents with the poor.
What is a Christmas Box?
The Christmas box was a wooden or clay container where people placed gifts.
To help the poor
An 'Alms Box' was placed in every church on Christmas Day, into which worshippers placed a gift for the poor of the parish. These boxes were always opened the day after Christmas, which is why that day became know as Boxing Day
A present for the workers
Many poorly paid workers were required to work on Christmas Day and took the following day off to visit their families. As they prepared to leave, their employers would present them with Christmas boxes.
During the late 18th century, Lords and Ladies of the manor would "box up" their leftover food, or sometimes gifts and distribute them the day after Christmas to tenants who lived and worked on their lands.
And the tradition still continues today ......
Christmas boxes
The tradition of giving money to workers still continues today. It is customary for householders to give small gifts or monetary tips to regular visiting trades people (the milkman, dustman, coalman, paper boy etc.) and, in some work places, for employers to give a Christmas bonus to employees.
A quick Google does wonders to display the wide variety of reasons why the 26th December is called "Boxing Day" but the Wiki version
http://en.wikipedia.org/wiki/Boxing_Day" onclick="window.open(this.href);return false; is close to what I believe but I'm sure Boxing Day was so called far longer ago than before the damned Colonials got their so called independence.