AMERICAN'S DON'T CELEBRATE IT! - BUT HERE'S AN EXPLANATION
Below are a couple of reasons and meanings given by wikipedia - I however like the explanation of boxing up old gifts for the poor!
From Wikipedia, the free encyclopedia
Though not an official holiday in the United States, some Americans use the term "Boxing Day", particularly those who live near the Canada – United States border. In Canada, Boxing Day is listed in the Canada Labour Code as an optional holiday. Only the province of Ontario has made it a statutory holiday where all workers receive time off with pay.
EtymologyThe exact etymology of the term "boxing" is unclear and there are several competing theories, none definitive. The tradition has long included giving money and other gifts to the needy and those in service positions. The European tradition dates to the Middle Ages, but the exact origin is unknown. Some claim it dates to the late Roman/early Christian era when metal boxes placed outside churches collected special offerings tied to the Feast of Saint Stephen.
A clue to Boxing Day's origins appears in the Christmas Carol, "Good King Wenceslas". Wenceslas, who was Duke of Bohemia in the early 10th century, was surveying his land on St. Stephen's Day, 26 December, when he saw a poor man gathering wood in the middle of a snowstorm. Moved, the King gathered up surplus food and wine and carried them through the blizzard to the peasant's door. The alms-giving tradition has always been closely associated with the Christmas season, but King Wenceslas' good deed came the day after Christmas, when the English poor received most of their charity.
In the United Kingdom, it is a custom for tradesmen to collect "Christmas boxes" of money or presents on the first weekday after Christmas as thanks for good service throughout the year. This is mentioned in Samuel Pepys' diary entry for 19th December 1663; and widely in Victorian literature.  Another possibility is that the name derives from an old English tradition: in exchange for ensuring that wealthy landowners' Christmases ran smoothly, servants were allowed to take the 26th off to visit their families. The employers gave each servant a box containing gifts and bonuses (and sometimes leftover food). In addition, around the 1800s, churches opened their alms boxes (boxes where people place monetary donations) and distributed the contents to the poor.
To protect shipsDuring the Age of Exploration, when great sailing ships set off to discover new land, a Christmas box was a good luck device. It was a small container that priests placed on each ship while still in port. Crewmen, to ensure a safe return, dropped money in the box. It was then sealed and kept on board for the entire voyage. If the ship came home safely, the crew gave the box to the priest in exchange for the saying of a Mass of thanks. The priest kept the box sealed until Christmas, and then opened it to share the contents with the poor.
To help the poorAn 'Alms Box' was placed in every church on Christmas Day, into which worshipers placed a gift for the poor of the parish. These boxes were always opened the day after Christmas, which may be why that day became known as Boxing Day.
A present for the workersDuring the late 18th century, Lords and Ladies of the manor "boxed up" leftover food, or sometimes gifts, and distributed them the day after Christmas to tenants on their lands. Many poorly paid workers had to work on Christmas Day and took the following day off to visit family. As they prepared to leave, employers presented them with these Christmas boxes.
The tradition of giving money to workers continues today. It is customary for householders to give small gifts or monetary tips to regular visiting trade people (the milkman, dustman, coalman, paper boy etc.) and, in some work places, for employers to give a Christmas bonus to employees.
DateBoxing Day is a secular holiday, but is traditionally celebrated on 26 December, St. Stephen's Day, the day after Christmas Day.
When the 26th December falls on a Sunday, traditionally Boxing Day is moved to the 27th December, supposedly to maintain Sunday church-going and other Sunday observance. In Britain, Boxing Day is usually celebrated on the following day after Christmas Day, which is 26 December. Like Christmas Day, Boxing Day is a bank holiday (public holiday) throughout Britain. When Christmas Day falls on a Friday or Saturday, Boxing Day, which is a bank holiday, is the following Monday.
In Ireland—when it was part of the United Kingdom of Great Britain and Ireland—the UK's Bank Holidays Act 1871 established the feast day of St Stephen as a non-movable public holiday on 26 December. Since the Irish War of Independence, the name "Boxing Day" is used only by the authorities in Northern Ireland (which remained part of the United Kingdom). There, Boxing Day is a movable public holiday in line with the rest of the United Kingdom.
In Scotland, Boxing Day has been specified as an additional bank holiday in Scotland since 1974, by Royal Proclamation under the Banking and Financial Dealings Act 1971. 
In the Australian state of South Australia, 26 December is a public holiday known as Proclamation Day and Boxing Day is not normally a public holiday. However, Canada, the USA, and many other countries use Boxing Day for commercial use. Items usually cost less and many sales are on. Traditionally people saved one of their gifts that was still wrapped and donated it to charity. Now it has turned into a much more commercial occasion, mainly for people to save money on many items.
CalendarIn the countries that observe this holiday, 26 December is commonly referred to both as Boxing Day and as St. Stephen's Day, no matter what day of the week it occurs. However, in some countries, holidays that fall on Saturday or Sunday are observed on the next weekday. Traditionally, Boxing Day could not be on a Sunday, that day being the officially recognized day of worship, so it was the next working day of the week following Christmas Day, (i.e., any day from Monday to Saturday). But in recent times, this tradition has been either forgotten or ignored. Most calendars and media outlets consider 26 December Boxing Day even when it falls on a Sunday.
If Boxing Day falls on a Saturday (as in 2009), then Monday 28 December is declared a bank or public holiday. In the United Kingdom and some other countries, this is accomplished by Royal Proclamation. In some Canadian provinces, Boxing Day is a statutory holiday that is always celebrated on 26 December. In Canadian provinces where Boxing Day is a statutory holiday, and it falls on a Saturday or Sunday, compensation days are given in the following week.
If Boxing Day falls on a Sunday, then Christmas Day is on a Saturday, so in countries where these are both bank or public holiday, the Statutory Holiday for Christmas is moved to Monday 27 December and the Statutory Holiday for Boxing Day is moved to Tuesday 28 December, as it has happened in 2010.
If Christmas Day falls on a Sunday, then Boxing Day is on Monday 26 December, and no Royal Proclamation is required. In such a circumstance, a 'substitute bank holiday in the place of Christmas Day' is declared for Tuesday 27 December, so the Boxing Day holiday occurs before the substitute Christmas holiday.
LegalityAlthough the same legislation—the Bank Holidays Act 1871—originally established the bank holidays throughout the United Kingdom, the day after Christmas was defined as Boxing Day in England, Scotland and Wales, and the feast day of St Stephen in Ireland. (Note that a 'substitute bank holiday in place of 26 December' is only possible in Northern Ireland, reflecting the legal difference in that St. Stephen's Day does not automatically shift to the Monday in the same way as Boxing Day.)
ShoppingIn Canada, New Zealand, the United Kingdom, and some states of Australia, Boxing Day is primarily known as a shopping holiday, much as the United States treats the Friday after Thanksgiving. It is a time where shops have sales, often with dramatic price decreases. For many merchants, Boxing Day has become the day of the year with the greatest revenue. In the UK in 2009 it was estimated that up to 12 million shoppers appeared at the sales (a rise of almost 20% compared to 2008, although this was also affected by the fact that the VAT would revert to 17.5% from 1 January).
Many retailers open very early (typically 5 AM or even earlier) and offer doorbuster deals and loss leaders to draw people to their stores. It is not uncommon for long queues to form early in the morning of 26 December, hours before the opening of shops holding the big sales, especially at big-box consumer electronics retailers. Once inside, the shoppers often rush and grab, trashing and pillaging as they go, as many stores have a limited quantity of big draw or deeply discounted items. Because of the shoulder-to-shoulder crowds, many choose to stay home and avoid the hectic shopping experience. The local media often cover the event, mentioning how early the shoppers began queueing up, providing video of shoppers queueing and later leaving with their purchased items. The Boxing Day sales have the potential for customer stampedes, injuries and even fatalities. As a result, many retailers have implemented practices aimed at controlling large numbers of shoppers, most whom are typically irate due to the cold (or, in Australia and New Zealand, hot) weather, and anxious for bargains. They may limit entrances, restrict the number of patrons in a store at a time, provide tickets to people at the head of the queue to guarantee them a hot ticket item, and canvass queued-up shoppers to inform them of inventory limitations.
In recent years, retailers have expanded deals to "Boxing Week." While Boxing Day is 26 December, many retailers who hold Boxing Day Sales will run the sales for several days before or after 26 December, often up to New Year's Eve. Notably, in the recession of late 2008, a record number of retailers were holding early promotions due to a weak economy. Canada's Boxing Day has often been compared with the American Super Saturday, the Saturday before Christmas. In 2009, a number of major Canadian retailers had their own Black Friday promotions to discourage shoppers from crossing the border.
In some areas of Canada, particularly in Atlantic Canada and parts of Northern Ontario (including Sault Ste. Marie), most retailers are prohibited by law from opening on Boxing Day. In these areas, sales otherwise scheduled for 26 December are moved to the 27th.
In Ireland, since 1902, most stores remain closed on St. Stephen's Day, as with Christmas Day. In 2009, some stores decided to open on this day, breaking a 107-year-old tradition. Some stores have also started their January sales on this day.
Cyber Boxing DayThe online version of Boxing Day has been referred to as "Cyber Boxing Day". In the UK in 2008, Boxing Day was the busiest online shopping day of the year. In 2009, many retailers with both online and High Street stores launched their online sales on Christmas Eve and their High Street sales on Boxing Day.
SportThe association of Boxing Day with sport in early village celebrations has led to the folk etymology that Boxing Day is traditionally associated with boxing, although the word box can mean a gift or gratuity, especially one given at Christmas, especially in Britain. In African Commonwealth nations, particularly, Ghana, Uganda, Malawi, Zambia and Tanzania, prize fighting contests are held on Boxing Day. This practice has also been followed for decades in Guyana and Italy.
In both England and Scotland, it is traditional for the Premier League and Scottish Premier League respectively, as well as the lower divisions and Rugby Football leagues, to hold a full programme of football and Rugby matches on Boxing Day. Traditionally matches on Boxing Day are played against local rivals. This was originally to avoid teams and their fans having to travel a long distance to an away game on the day after Christmas Day. It also makes the day an important one in the sporting calendar.
In horse racing, there is the King George VI Chase at Kempton Park Racecourse in Surrey. It is the second most prestigious chase in England, after the Cheltenham Gold Cup.
Boxing Day is a popular day in the UK and United States for mounted fox hunters. Despite fox hunting being banned by the Hunting Act in 2004, Boxing Day remains the biggest hunt of the year for most hunts in the UK by use of scent drag trails instead of live game.
Australia holds the first day of the Boxing Day Test in Melbourne at the Melbourne Cricket Ground and the start to the Sydney to Hobart Yacht Race.
The IIHF World U20 Championship (ice hockey) typically begins on 26 December. In Canada, the tournament is one of the largest sporting events of the year, often drawing comparisons to the Super Bowl in the United States.
The NHL tends to have close to a full slate of games (11 will be played in 2010), following the league-wide days off given for Christmas Eve and Christmas Day.
The Spengler Cup (ice hockey) also begins on 26 December in Davos, Switzerland and includes HC Davos, Team Canada and other top European Hockey teams.