The Twelve Days of Christmas and the associated evenings of those twelve days (Twelve-tide), are the festive days beginning the evening of Christmas Day (December 25) through the morning of Epiphany (January 6). The associated evenings of the twelve days begin on the evening before the specified day. Thus, the first night of Christmas is December 25–26, and Twelfth Night is January 5–6. This period is also known as Christmastide.
Over the centuries, differing churches and sects of Christianity have changed the actual traditions, time frame, and their interpretations. St. Stephen's Day, for example, is December 26 in the Western Church and December 27 in the Eastern Church. December 26 is Boxing Day in the United Kingdom and some of its former colonies; December 28 is Childermas or the Feast of the Innocents. Currently, the 12 days and nights are celebrated in widely varying ways around the world. For example, some give gifts only on Christmas night, some only on Twelfth Night, and some each of the 12 nights. What remains constant is celebrating the birth of Jesus on December 25th, and a period of twelve days and nights leading up to Epiphany.
-Wikipedia, 12 days of chirstmas