The Twelve Days of Christmas, and the associated evenings of those twelve days (Twelve-tide), are the festive days beginning on Christmas Day (December 25) through to the evening of the Twelfth Day of Christmas, (January 5). Thus, the first night of Christmas is December 25–26 and Twelfth Night is January 5-6. This period is also known as Christmastide. The day after Twelfth Night is Epiphany on January 6.

Over the centuries, differing churches and sects of Christianity have changed the actual traditions, time frame and their interpretations. St. Stephen's Day (or Boxing Day), for example, is December 26 in the Western Church and December 27 in the Eastern Church. Boxing Day, the first weekday after Christmas, is observed as a legal holiday in parts of the Commonwealth of Nations and was traditionally marked by the giving of Christmas boxes to service workers (such as postal workers and trades people) in the United Kingdom; 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.