The Twelve Days of Christmas are the festive days beginning on Christmas Day (25 December). This period is also known as Christmastide. The Twelfth Day of Christmas is 5 January - the day before the Feast of the Epiphany on 6 January.
In England in the Middle Ages, this period was one of continuous feasting and merrymaking, which climaxed on Twelfth Night, the traditional end of the Christmas season. In Tudor England, Twelfth Night itself was forever solidified in popular culture when William Shakespeare used it as setting for one of his most famous stage plays, titled Twelfth Night. Often a Lord of Misrule was chosen to lead the Christmas revels.
Many in the United Kingdom and other Commonwealth nations still celebrate some aspects of the Twelve Days of Christmas. Boxing Day (26 December) is a national holiday in many Commonwealth nations, being the first full day of Christmas. Victorian era stories by Charles Dickens (and others), particularly A Christmas Carol, hold key elements of the celebrations such as the consumption of plum pudding, roasted goose and wassail. While these foods are consumed more at the beginning of the Twelve Days in the UK, some dine and dance in the traditional way throughout, all the way to Twelfth Night.
Nowadays, the Twelfth Day is the last day for decorations to be taken down, and it is held to be bad luck to take decorations down after this date. This is in contrast to the custom in Elizabethan England, when decorations were left up until Candlemas; this is still done in some other Western European countries such as Germany.
With the onset of more Americanized and secular traditions throughout the past two centuries (such as the American "Santa Claus"), also the popularity of Christmas Eve itself as a holiday, and rise in popularity of New Year's Eve parties as well, the traditions of the Twelve Days of Christmas have been largely forgotten in the U.S. This is also heightened by the commercial practice to have "After-Christmas Sales" begin on 26 December and run usually until New Year's Eve. Indeed, contemporary marketing and media tend to espouse the (erroneous) belief that the Twelve Days end on Christmas and thus begin 14 December.
However, a small percentage of Christians of many sects have held on to their own favorite ways to celebrate and those who choose to also have their own church to guide them in a spiritual way of marking this reverent holiday. Americans who celebrate various ways include secular Christians of all backgrounds, religious Catholics, Orthodox Christians, Lutherans, Episcopalians, Moravians and those of the Amish and Mennonite communities.
Today, some celebrants give gifts each of the Twelve Days, feast and otherwise celebrate the entire time through to Epiphany morning. Lighting a candle for each day has become a modern tradition in the U.S. and of course, singing the appropriate verses of the famous song each day is also an important and fun part of the American celebrations.
Some still celebrate Twelfth Night as the biggest night for parties and gift-giving and some also light a Yule Log on the first night (Christmas) and let it burn some each of the twelve nights. Some Americans also have their own traditional foods to serve each night.
As in olden days, Twelfth Night to Epiphany morning is then the traditional time to take down the Christmas tree and decorations.
(All information comes from the wikipedia entry located at http://en.wikipedia.org/wiki/Twelve_Days_of_Christmas )