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.

The 12 days has its origins in the pre-Christian Scandinavian Winter Solstice, in which the Midwinter Feast lasted at least twelve days, hence the twelve days of Christmas.